Fix to skip solution folders

Hi Mike,

I had fixed an issue I was using your great module.

Basically if the solution contains any solution folder (existing or not) it marked to fail the task as there is a file read call against a folder (with it not possible) or a non existing path (which is not possible neither).

I added a filter function to check if path exists and that is the case then verify if it is not a folderm.
This way any existing folder or a non existing one will be skipped when processing the solution file.

Would you mind to review and merge.
Also if possible would be great if published in the npm registry also.

Let me know if you have any question.

Thanks in advance.
Nestor.