Added a CommandWithTerm Class

Added new CommandWithTerm Class similar to the CommandWithMeta Class.
Designed to reusable for different object types. Currently this is draft and I am still testing it. Just wonder if I was on the right track with this solution. Feedback is requested. I will write tests when I know that I am on the right track. 

Related #1379
We should have tests where we expect success here too. You can associate a custom taxonomy with users via a run-time plugin. Check this out as an example: https://github.com/wp-cli/wp-cli/blob/master/features/help.feature#L35