Fix example cfg
Port CLI to click
Add click to deps
Make logging compatible with click's testrunner
Support for default command
Don't depend on vdirsyncer logging code

And avoid random breakage if user has a "wrong" version of vdirsyncer
installed.
Add some tests.

Also remove default subcommand feature because of
https://github.com/mitsuhiko/click/issues/205
allow empty color strings

also default color is now '' (empty string)
fix: controllers.agenda should accept empty sets