List khal items as org items.

Agenda items from `khal` can be converted to org items using the `khalorg list`
command. For example:

khalorg list my_calendar today 90d > my_calendar.org

Here, the `khal` agenda items of the calendar `my_calendar` are converted to
org format and written to a file called `my_calendar.org`. The range is
specified from `today` till `90d` (90 days) in the future.

If ~khalorg list --format~ option is not defined, the default one is used
which can be found at `./khalorg/static/khalorg_format.txt`. If you want to
define your own format, you have 2 options: you can use the
`khalorg list --format` option, or you can place your custom format at
`$HOME/.config/khalorg/khalorg_format.txt` this format will then be used
instead of the default.

The following keys are supported:
- attendees     - calendar
- categories    - description.
- location      - organizer
- rrule         - status
- timestamps    - title
- uid           - url
