Better excluded files filter

Now if i pass a single file to CLI csslint will recursively find all files in excluded dirs. In my projects i have a lot of 3rd party libraries which i don't want to lint. So to lint one file i can have about ~1000 files in excluded. This is not optimal from the performance side.

I propose to use function `isExcluded` before linting files. It's simple & fast.

Also i added `grunt-cli` to `package.json`.