Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
It's difficult to test for what is/is not a valid URL (e.g. you do not account for URLs with embedded username/password). It is much easier to test if something begins with a valid URI scheme/protocol. Use this regexp for testing `/^[a-z][-+.a-z]*:/i` (see RFC 3986) everything else can be considered a URL with a missing `http://` part.

Also, use `regexp.test(str)` to perform tests if you don't need to capture the match.