Label format config

Implements #85 (closed)

Open tasks:

• a dry mode for _unify_labels which only predicts the changes without applying them
• a conflict resolving method:
  • should default to leave the label untouched (or upon addition abort)
  • should be configurable to support conflict resolution via suffixes like _1, .1, a, _a, etc. (counting upwards ofcourse)
• changelog and docs