Support for date shortcuts

I use khal frequently on the command line, so I have aliased it to k in my shell. But navigating around the calendar or agenda with giving the full date is a bit cumbersome, I always have to type so much ;-) It would be of great help to have khal <date> as a shortcut to print the agenda for that date.

Then especially useful would be more date formats, e.g. khal 23rd would list the events at 23rd of the current month (maybe related to #169).

To be honest, my intended use of khal is heavily influenced by my usage of taskwarrior, which allows to filter tasks easily by giving a filter expression as the first parameter, and the "show events from this date" feels much like such a "filter". The taskwarrior manual has a long list of useful synonyms and date abbreviations, some of which could be useful for khal also.

Review khal’s existing command-line date and agenda handling, then compare the requested shortcuts with the taskwarrior date names linked in the issue. Define which date and ordinal formats are supported and how current-month resolution works; done means khal <date> reliably prints that date’s agenda.