Added enable and disable breakpoint methods.

Per the enhancement request from https://github.com/linkedin/archetype/issues/99
one final comment...

could you change the warning prefix to `[#{Archetype.name}:breakpoint:#{friendly_method}]` so it reads `[archetype:breakpoint:enable]` etc

and could you also add these warnings to `WARNINGS.md` (or just add a placeholder with a TODO so that I remember to do this one when I get around to #70)