Allow setting gvimrc and default to NONE

Hey,

I was writing tests and noticed `~/.gvimrc` was still getting sourced. Passing `-U` to Vim solves this.

I'm terribly sorry for not adding tests, but seeing that you use RSpec, I was afraid to run them because of the [kernel panics](https://github.com/AndrewRadev/vimrunner/issues/29). :)