Enhance pod lib lint to support multiple Spec files in the folder

This PR updates the pod lib lint command to support multiple spec files in the same folder, it currently raises if more than one pod spec is present, and doesn't take a command line argument like pod spec lint does.

This is quite useful when one forks a pod. The current behavior of requiring a single pod spec file in the current folder forces forked pods to remove (or entirely replace) the original pod file, which can create a big mess when sending PR to the original project.
The other option is to skip pod lib lint on builds, but you're the opening yourself to releasing completely broken pods.

Supporting multiple pod specs makes it possible to copy the original pod spec, rename the pod and push to a different Pod repo.
It's typically what we do when we need to fix something in third party pods, we append -OpenTable to the pod name and push it to our own repo.
It also makes it quite easy to revert back to the original pod, by simply dropping the suffix in the Podfile's dependencies.

Anyway, I've been careful to add some tests this time :)

Let me know if I should change anything

/kra