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 believe I can do this pretty easily, because it is already doing it. It is doing an explode on a commas. I can implement it as you wish. I just change 

```php
public function add( $args, $assoc_args ) {
		list( $object_id, $taxonomy, $term ) = $args;

		$this->set_obj_id( $object_id );

		$this->taxonomy_exists( $taxonomy );

		$terms = explode( ",", $term );

		$result = wp_set_object_terms( $object_id, $terms, $taxonomy, true );

		if ( ! is_wp_error( $result ) ) {
			WP_CLI::success( "Added term." );
		} else {
			WP_CLI::error( "Failed to add term." );
		}
	}
```

To
```php
public function add( $args, $assoc_args ) {
                $object_id      = array_shift( $args );
                $taxonomy     = array_shift( $args );
                $terms           = $args;

		$this->set_obj_id( $object_id );

		$this->taxonomy_exists( $taxonomy );

		$result = wp_set_object_terms( $object_id, $terms, $taxonomy, true );

		if ( ! is_wp_error( $result ) ) {
			WP_CLI::success( "Added term." );
		} else {
			WP_CLI::error( "Failed to add term." );
		}
	}
```

What do you think? Should I change the tests as well?