Add --category to dunstify

[bynect]

I am not sure if the difference with notify-send for -c is worth keeping just for printing capabilities...

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.19%. Comparing base (b7abe8f) to head (c3db473).
Report is 96 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1429   +/-   ##
=======================================
  Coverage   65.19%   65.19%           
=======================================
  Files          50       50           
  Lines        8729     8729           
  Branches     1034     1034           
=======================================
  Hits         5691     5691           
  Misses       3038     3038           

Flag	Coverage Δ
unittests	65.19% <ø> (ø)

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[fwsmit]

I don't think the capabilities printing is used a lot in scripts, so it's okay to change the behavior