Enhancement: Added the ability to give the arrows a tooltip.

Hi there,

I've added the ability to pass an extra trailing text at the end of the arrow line by appending a :comment, so the arrow now accepts lines like this:

A->B:text:comment
A<->B:text:comment
A<-B:text:comment

It would be nice if this can be merged into your master branch, otherwise, I still thank you for the plugin :)