Add support for settings.RICHTEXT_FILTERS

I inquired about the possibility of this change [on the news group](https://groups.google.com/forum/#!topic/mezzanine-users/JksrFFGnfxQ), but found only crickets.  So here's a pull request instead.

``RICHTEXT_FILTER`` is convenient, but not flexible for multiple reusable apps to leverage.  While it's simple to write my own filter to rule them all, and import/execute my third-party filters, I believe this is more Django-like, the way middleware and context processors work.

This update adds support for a plural list-based ``RICH_TEXT_FILTERS``, whose purpose is to provide an ordered sequence of filters:

```python
RICHTEXT_FILTERS = (
    'mdown.filters.codehilite',
    'gist_embed.filters.gist_embed',
)
```

Underneath, ``RICH_TEXT_FILTER`` is a fallback if the plural version is empty, so everything works as it did before.

For consistency, an alias for the ``richtext_filter`` template filter has been added to match the plural nature of the new setting: ``richtext_filters`` simply calls on ``richtext_filter`` for its output.  Not sure how you would prefer to handle that, but maintaining backwards compatibility with existing use of ``richtext_filter`` seems the best idea, I'm sure you agree.