Auto-Comment continue on next line

I've been using this fantastic plugin for a while, but there's bunch of features I sorely missed.
Automatic comment continuation was one of them, if this was skipped due to an oversight and not because of a conscious decision, then here's a pull that adds this feature.

Tested with:
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled May 12 2014 22:27:13)
MacOS X (unix) version
Included patches: 1-273
Compiled by Homebrew