Added the ability to specify `true` for the `jshintrc` option

I personally prefer to store my config in `.jshintrc` files so that my config can be shared between grunt and my IDE. As my project becomes a bit more complex, and different directories are using different rc-files, I thought it would be nice to have the option to just defer rc-file lookup to jshint allowing me to "set and forget" the jshint config in my Gruntfile. 

It also presents identical results when I run `grunt jshint`, `jshint .`, or check a single file against jshint in my IDE.