Update documentation links for Lua/APR binding

Bug fix: Recognize default stylesheet in lxsh.formatters.html.preamble()

Lexer & syntax highlighter for shell script code

Refactor lexer and highlighter initialization (in preparation for ...)

Syntax highlighter for BibTeX source code

BibTeX lexer contributed by Brendan O'Flaherty

I made the following modifications:

 * Case insensitive entry name matching using the ic() function
 * The string literal pattern now includes end of line characters
   because http://en.wikipedia.org/wiki/BibTeX#Examples gave me
   the impression that string literals can span multiple lines

Track line/column numbers in lxsh.lexers.*.gmatch()

Support for multiple output formats (HTML, LaTeX, RTF)

Documentation links for LuaSocket, LuaFileSystem & Penlight.

Refactor documentation link handling

The Lua lexer previously always matched expressions like "io.input" as
one identifier. This was a quick and dirty hack to add support for
documentation links in the Lua highlighter. I've decided to keep the
hack because it can save quite a lot of code however this behavior is
now disabled by default and can be enabled with join_identifiers=true
(the Lua highlighter of course enables this behavior by default).