add ability to directly subscribe to feeds

[ossilator]

google's "RSS Subscription" extension (fork) offers the ability to submit the extracted feed url to an aggregator service chosen from a configurable list (but is no good for github without manually adding the .atom extension). it would be nice if your extension also offered this option. i would use it with Tiny Tiny RSS or a local cgi script that calls rss2email.

the fork mentioned in #27 sends the url to another extension instead, which is somewhat complementary.