Provide mw command

Summary:
MediaWiki ships with maintenance PHP CLI scripts,
requiring configuration too. This wrapper provides
required environment to allow MediaWiki SaaS to prepare it.

Usage: mw <wiki database name or alias> <script to run>

This release doesn't currently allow to run other scripts,
like the ones provided by extensions.

Test Plan:

• mw script shell
• mw script /tmp/WikimediaMaintenance/listDatabases.php

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1452