Add a theme section in the configuration file to allow users to
set the colors used by different elements of the ui.
Improve the update mechanism so that incremental changes to the
configuration file can be incorporated.
* only bibtex format is supported
* all tests except test_repo.py and edit test pass
* edit and update commands were not updated
* removed --format argument from export, only bibtex is supported.
Fabien Benureau
Olivier Mangin