* khal: fix contact integration
- Add tests for contact+khal
- Make options `color`/`priority` available for contact accounts
* khal: add separate calendar for each contact collection
A contact account may have multiple VCARD collections, but Khal doesn't
search recursively. Collection folder names must be hardcoded, and each
has its own calendar.
- Add khal.collections option for contact accounts
- Default to previous setup for accounts with a single collection
- Add tests
* khal: specify how priority is defined by Khal
See https://khal.readthedocs.io/en/latest/configure.html
