Linter for files in gitignore

This was first raised in #39 
The idea is to add a `gitignore` field to the language object that will contain the files that are supposed to be not pushed to the repo due the fact that are in the `.gitignore` file

``` js
{
   name: 'language name',
   files: {
   ...
   },
   gitignore: {
   ...
   }
}
```

Then there would be a separate linter to perform the checks
