Option to generate docs from local grunt-docs repo for offline work.

Hey @vladikoff, 

I was working on the mini-badge document yesterday without internet access and in order to preview my changes I added an option to the Gruntfile that allows you to use the local grunt-docs repo as a source to generate the docs from scratch if you set it to true. It assumes that grunt-docs lies in the same folder as the gruntjs.com repo.

Cool?

PS.: also changed double quotes to single quotes cause the linter was :cry: