Add support for reading and walking through content of a whole directory structure

[Porkepix]

Currently it isn't possible to provide one or more directories and to run through the whole directory tree to test every Ansible's file.
It would be a nice addition to be able to run it by providing directories and recursively browse them.
Filed the issue here as ansible-review seemed to be the most fitted for it, but maybe this could also be in ansible-lint.