Add basic support for vim-commentary

[Hultner]

Vim-commentary is a incredible useful and common plugin which is implemented in many other vim-emulators. It basically allows me to comment a motion e.g. gcap to comment a paragraph or gcc to comment current line. I use this a lot and find myself trying to use it by muscle-memory all the time in XSVim.

[nosami]

Yeah, I can add this. Should be easy enough.