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 
Should we also support the arguments `wp_get_object_terms()` supports?

http://codex.wordpress.org/Function_Reference/wp_get_object_terms#Argument_Options

You could support all of them in a generic way with:

```
* [--<field>=<value>]
* : One or more args to pass to wp_get_object_terms().
```