Add support to concat symlinks

Include support for broccolijs/broccoli-merge-trees#12

@rlivsey, @rjackson a review would be welcomed
I think, the problem was that `readFileSync` does not read symbolic links. Therefore, `lstat` identifies symbolic links to call `readlinkSync` in these cases.

I am using this PR for an app, let me know if you want me to report or test anything.