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
I didn't ignore you when you posted this before. I just don't agree with this feedback. I will refer you to my reply earlier. https://github.com/wp-cli/wp-cli/pull/1443#discussion_r18750278
Requiring taxonomy names make this method MUCH less useful.