ability to write per-feed and global filters using regexes

[fazalmajid]

/degunk.py and /transforms.py provide functionality to strip out ads and other junk, but that requires coding the regexes in a Python file and restarting the server, which is not particularly user-friendly. We should provide a mechanism that allows regex-based filtering (possibly with a user-friendly front-end) and the ability to preview the effects of a filter against an article.