`add_feed` with regex characters breaks rewrite rules

[soulseekah]

add_rule( 'test.json', ... ) does not get escaped with preg_quote. Fine for this example, but something like add_rule( 'test[json]', ... ) will definitely yield unexpected results. Something like add_rule( 'test???', ... ) will break the whole feed (maybe even more) regular expression.

[pento]

Patch needs unit tests and reviewing.