Allow ansible (ad-hoc) to support --extra-vars

Currently the `ansible` command does not allow the use of `--extra-vars`.  This has been a frequent request in IRC recently.  I see no technical reasons why this wasn't included.

This PR adds `--extra-vars` support to the `ansible` command and moves the `--extra-vars` option to `ansible.utils.base_parser` and moves the now common `extra_vars` parsing into `ansible.utils.parse_extra_args`

A common use case is to override `ansible_python_interpreter`.