Add subdirectory exclude support

(This is somewhat of a hack, open to suggestions).

With our codebase we need to exclude subdirectories (foo/bar/baz). Currently PHP-Parallel-Lint only supports excluding top level directories. This PR adds support for excluding subdirectories.

I had to add "." in getPathname due to Unix relative directories being listed as './foo' instead of just '/foo'. I don't have a Windows machine to verify it works there as well. (Or if this is even the best method.)

I'm fairly unfamiliar with the Iterator PHP stuff, so I'm open to hearing better ways to do this.