Added a CommandWithTerm Class (Version 2)

Added new subcommands wp user term and wp post term. 

Created a new pull request to follow contribution guide lines. 

Related #1474 
> I have used time to get to this point and now you are adding new things.

A pull request is not done until it's done. `<term>...` is an existing pattern used elsewhere in WP-CLI (e.g. `wp plugin activate <plugin>...`). Supplying arguments as CSV is not. I apologize for implying the PR was done. At this point, it looks pretty close to shipping.

> What the hell?

Please watch your language. A good rule of thumb for behavior is to treat others as you'd like to be treated. I'll only ask once.