Extent the file-system grunt library

New functions available:
- `rmdirRecursiveSync` to remove a directory and all its content synchronously
- `copyFile` to copy a file asynchronously

If and once this pull request and https://github.com/concrete5/concrete5/pull/1495 are merged, I've already created the grunt task to replace short tags with long tags :wink:, and I'll create a new pull request