Add hotkey support, check URL when adding new feed

Add hotkey support for +/add new feed, -/remove feed, and F5/refresh
feeds.

Rename functions addNewFeed to addFeed.

Add functions for key binding refreshRSSBind, addFeedBind, and
removeFeedBind, all act as a portal to the actual funcs.

Add basic error checking to make sure URL entered when adding new feed
is not empty.