dim: turn commit-add-tag into a proper dim subcommand

This lets you add e.g. review tags to a commit on the command line:

$ dim commit-add-tag "Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com&gt;">

This could be used as a building block for further scripting.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>