feature: add khal calendar

[gvolpe]

Currently failing to build, not sure why:

       … while calling 'genCalendarStr'

         at /nix/store/6zmjz22khxi92dh43w1dp2mjadysjjy8-source/modules/programs/khal.nix:27:26:

           26|
           27|   genCalendarStr = name: value:
             |                          ^
           28|     concatStringsSep "\n" ([

       error: value is null while a set was expected