Add text filters

Add two filters: markdown_thumbnails, html_thumbnails. Those filters use
regex to replace images with thumbnails. The width of the thumbnails is
set to THUMBNAIL_FILTER_WIDTH which is by default 500.

#### Log from the test run:
```
Running tests for 'settings.pil'
Creating test database for alias 'default'...
..
----------------------------------------------------------------------
Ran 2 tests in 0.521s

OK
Destroying test database for alias 'default'...
```
I commented out the other tests while running the test suite since they do not work at the moment.