prune-config-settings

- WORK IN PROGRESS.
- continue default behavior of pruning merged branches
- respect new should_prune setting if it exists in YML file
- override should_prune setting in YML file via --no-prune 
  or --prune flag on release.

Questions: Does ```method_option :prune, :type => :boolean, :force => true, :aliases => '-p'``` make the prune option default to true if not provided?

- Should I call the param something else instead of should_prune? 
- No idea how I should write tests for this. Suggestions?
- is my logic correct?