Add a way to temporarily disable line numbering.

This has not been tested, and is probably done in a stupid way since I do not know vimL.

But the reasoning behind this is that I sometimes have to copy stuff out of vim to another terminal or a browser, and the line numbers get in the way.

I know you can just do :set nonumbers and :set norelativenumbers, but I figured it would be nice to be able to switch between having absolute, relative and no line numbers with the plugin.