Improve filters: fix SuspiciousOperation in regex filters

This is the same as #256 just moved it to a branch on the this repo.

>If the path is prefixed with MEDIA_URL it will be replaced with MEDIA_ROOT (updated in 1b0ba0d)

>Related to #251
In [markdown_thumbnails](https://github.com/mariocesar/sorl-thumbnail/pull/284/files#diff-03ede78d3f0e52c9639c89e2703fa106L265) and [html_thumbnails](https://github.com/mariocesar/sorl-thumbnail/pull/284/files#diff-03ede78d3f0e52c9639c89e2703fa106L271)