Use GArray for user_defined_input_rects

Better to do this than realloc, so in case the `length` property of the
JS array object, there won't be memory corruption.

Use constant format strings for all printfs

To avoid accidental collision of the input with format specifiers like
%s.

Merge branch 'async-js-api'

Conditionally compile support for WebKit settings

Because https://travis-ci.org/github/anko/hudkit/builds/739360282

all you fancy LTS distros with old libraries and stuff grr

Readme: Add radness

Fix positioning when started on secondary monitor

Previously, we were treating the desktop as an uninterrupted uniform
plane, and optimistically assuming that GTK will always stick our window
at absolute coordinates 0,0 when we create a window.

This assumption does not match reality: by default, GTK positions new
windows in the top-left corner of the currently active physical monitor.
This caused the overlay window to be misplaced when hudkit was started
on a monitor other than the top-left-most one.

Fixes #2

Update readme with some more tutorial