Add commands for editing config.json

[cmccandless]

Currently, whenever an exercise is added, removed, deprecated, etc in a track, the contributor must make the required changes by hand, which often results in errors or inconsistency. I propose adding the following functionality to configlet:

• add
  • Creates a new entry in config.json for the specified exercise
• edit
  • Makes changes to existing exercise entry in config.json
• remove
  • Remove specified entry from config.json
• deprecate
  • Marks the exercise as deprecated in config.json

[cmccandless]

RE: exercism/python#1084 per suggestion from @kytrinyx

[cmccandless]

Some work was done in exercism/python#1089 towards this end. While the implementation is in Python (not Go), there may be something worth salvaging from that PR.