comply with XDG Base Directory Specification (fixes #13)

User config tree now defaults to ~/.config/rapport, and
reports dir to ~/.local/share/rapport/reports.  Added a
new unit test for the latter.

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
Merge pull request #14 from aspiers/xdg-compliance

Let's have it merged then. I'll add another commit to keep the old config locations with lower priority. If necessary, migration code can still be written later on
add some useful custom filters
revamp github plugin
fix github plugin to handle rel="prev" links gracefully