Adding diff command.

[grasmash]

I have some reservations about merging this:

1. This command (as well as the merge and split commands) is significantly more complex than the existing commands.
2. This command feels a bit more niche and less universally useful.
3. The command doesn't seem to work entirely as intended. Comparing https://github.com/grasmash/yaml-cli/blob/diff-command/tests/resources/good.yml against https://github.com/grasmash/yaml-cli/blob/diff-command/tests/resources/good_also.yml does not register the different in inline-array.

@jmolivas @enzolutions

• Would you be open to leaving diff, split, and merge in Drupal console and using this library (yaml-cli) only for the existing commands?
• Are the diff, split, and merge commands used internally in Drupal console, or are they used by the community as well?