Support `--[no-]recur` flags

Heya! Hopefully this isn't too pedantic, but it's always been an English Gotcha for me that ack can't handle "[recur](http://www.merriam-webster.com/dictionary/recur)" and only accepts "[recurse](http://www.merriam-webster.com/dictionary/recurse)" as a flag, so this PR adds duplicate flags for `--[no-]recur` and updates documentation to support both.