Added URL option where possible

Terminal-notifier and growlnotify for windows are the only ones that supports this outright.

Also, growlnotify 2 supports it with a --url option. But that would require a way to detect whether v1.3 or v2 is installed.

Any ideas on how to do that? Not sure what the best practices are for nodejs scripts.

I was thinking I could write a method that would call growlnotify -v as a child process and parse its output to differentiate between v1.3 and v2.

Note: growlnotify 2 only works with Growl 2.0 and newer, which is a paid app :(