Update core/model/modx/modx.class.php

It would allow developers to create different parsers or extend the current
parser at ease without hacking the core.
Allow modX::getParser() to get an extended modParser instance

- slight alteration to PR
- add changelog entry
Allow modX::getParser() to get an extended modParser instance

Merge branch 'extend-parser' into release-2.2

* extend-parser:
  Allow modX::getParser() to get an extended modParser instance
  Update core/model/modx/modx.class.php