Introduce a new configuration parameter 'show_all_days'

If False (the default), khal shows only the days where there is at least
one event in 'calendar' and 'agenda' displays. If True, all days are
shown, even if there is no associated event.
Default value for show_all_days configuration parameter
Make the existing tests pass.
Add test case for show_all_days parameter
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Style nit
As requested, add my name for my tiny contribution