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
But you are keeping it consistencies to taxonomy, but it is more similar to [meta](https://github.com/wp-cli/wp-cli/blob/master/php/WP_CLI/CommandWithMeta.php#L20). I also don't really want to make this method less useful just for consistencies sake.