fix: controllers.agenda should accept empty sets
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
Don't reset colors if the string isn't colored