Use `.gitignore` file if there is `.jshintignore` file

I'm not sure if you will agree with this change but it makes sense to me...

Currently, `jshint` will read a gitignore file if explicitly specified (`jshint --exclude .gitignore`), but otherwise will not. I propose that, in the absence of a `.jshintignore` file, a `.gitignore` file should be used by default.