Add possibility for Podfile.local

We wanna have a local developer only Podfile to handle for example this case:

Normally all developers should use the project specific Podfile which is in our git repo. But if a single developer now wants to work on a dependency of that project, we wanna create a "local" Podfile which is include in the .gitignore. In this local Podfile the developer can then for example specify something like this:

pod "blabla", :local => '......'