Performance & git worktrees

Not many changes here, but a nasty bug was fixed, worktrees are handled
well now and performance of adding a build went up by an order of
magnitude or so (depends on GCC version in use, project size and how
coverage is collected).

General
-------

* Create index on files table of the database.  This bumps database
  version even though its schema doesn't actually change.  The change
  grows database size a bit, but makes addings new reports significantly
  faster.

Core and CLI
------------

* Handle git worktrees (config and database are looked up first in
  parent's `.git` now).
* Use `"current_working_directory"` key of gcov's JSON.
* Take advantage of `--stdout` option of gcov if present.  `new-gcovi`
  works significantly faster thanks to it and with less I/O.
* Fixed one use-after-free.

Vim-plugin
----------

* Update Vim plugin for newer fugitive API.

Building
--------

* Remove `-Werror` from `Makefile`.
* Make it work better with clang.

uncov.ini, `gcov --json-format`, `uncov help`

General
-------

* Pin version of the license to be AGPLv3 only.

Core and CLI
------------

* Add `help` builtin command which details commands and their usage.
* Parse `.git/uncov.ini` configuration file for settings.
* Implement name-binning of gcov input files to work around its bug.
  This fixes use of `new-gcovi` with GCC 8.
* Support `gcov --json-format` by `new-gcovi`, which makes it usable for GCC
  9 and newer.

Web-UI
------

* Make UI look better on smaller screens.
* Fix links to builds having number separators.

uncov-gcov
----------

* Add `--cpp-dtor-invocations` option (by Fabian Wermelinger).
* A slightly more robust parsing of `.gcov` files (by Fabian Wermelinger).
* Parse `.gcov` line data based on line number ID (by Fabian Wermelinger).
  This fixes incorrect coverage results on explicit template instantiation.
* Make --exclude and --include accept list of arguments (by Fabian Wermelinger).

Vim-plugin
----------

* Switch to using newer fugitive API.
* Improve startup time.

Building
--------

* Add install `PREFIX` to Makefile (by Fabian Wermelinger).
* Add `ld_extra` variable for linker to Makefile (by Fabian Wermelinger).
* Always use UTC timezone in the tests.

Intermediate gcov format and other improvements

Core and CLI
------------

* Add `new-gcovi` subcommand that uses `gcov`'s intermediate format and
  thus works faster than `uncov-gcov`.
* Better normalization of relative paths.
* Better parsing error messages for `new` subcommand.
* More strict input parsing for `new` subcommand.
* Make `new` subcommand accept reference with spaces.
* Fix incorrect parsing of optional build ids in subcommands.

Web-UI
------

* Add line numbers to diffs.  Clicking on them opens corresponding
  files.
* Add map of missed ranges for faster overview of what's not covered.
* Add LSB header to the sample init script.
* Put links in tables on first column instead of on whole row.
* Disallow robots from bothering the site (it doesn't contain
  interesting stuff).
* Highlight current line number.
* Fix formatting of line number anchors (avoid separators).

uncov-gcov
----------

The script is now deprecated in favour of `new-gcovi` subcommand.

* Add "WIP on " prefix to captured builds.
* Ignore `};` lines.
* Do not ignore lines starting with `static_cast`.
* Account for possible `*` in count column of `gcov`-files produced by
  new `gcov`.
* Fix `uncov-gcov` failing to restore untracked directory.

Vim-plugin
----------

* Make `:Uncov` command accept `@-buildid`.
* Fix error on folding last lines of a buffer.
* Fix coverage signs being offset by one.

Building
--------

* Build web object files from correct source files.
* Build with `-std={c=>gnu}++11` language option.
* Try to avoid file-system races on parallel build.
* Fix compiling tests against Boost 1.55.

Add Web-UI

Core and CLI
------------

* Add "regress" subcommand.
* Add @Branch notation to specify build.
* Add relative build addressing via `@-N` form.
* Make sure there are no separators in line numbers.
* Small speed improvement in diffs.
* Allow one extra coverage line compared to source file.
* Fix folding on colorless printing.
* Fix buildid resolution for "files" and alike.

Web-UI
------

Added it.

Capable of:
 * browsing files;
 * displaying current coverage;
 * displaying changes;
 * providing badges.

uncov-gcov
----------

* Add --ref-name option to uncov-gcov.
* Provide manual page.
* Make it possible to specify location of gcov files (--collect-root
  option).
* Fix a typo in usage message.

Vim-plugin
----------

* Accept buildid in :Uncov command.
* Ensure current line is not folded.
* Make :Uncov operate differently in uncov-buffer.
* Fix table of contents in Vim's documentation file.

Documentation
-------------

* Add complete Doxygen documentation.
* Better structure of the README.
* Make args in synopsis bold in the manual page.
* Fix diff screenshot in the README being blurry.
* Fix what INSTALL says about build mans.

First release.

Already capable of:

 * Code highlighting.
 * Comparison of coverage.
 * Displaying parts of files that need attention with regard to coverage.
 * Displaying coverage right in Vim.