Add possibility to selectively disable update of particular pods on 'pod insall'

[delebedev]

Sometimes, the pod spec is not actual or the code has issues and the developer has two options:

1. Fork the lib, fix code issues and change source (rather slow)
2. Hot-fix the code in the Pods directory (dangerous but fast)

The problem is when 'pod install' is exectued the changes made in option 2 are discarded. I propose to add an option to Podfile to skip update of pod source, f.e.

pod 'MyAwesome', skip_update:true

[fabiopelosin]

That would create confusion among users, especially in distributed teams. You can just copy the folder of the Pod to another location and use the :path option to preserve the changes. Which is more clear and just requires and additional minor step.

[supermarin]

+1 , @garnett if you change the pod source, probably you want to keep it locally in your project.

That way you can also share it with others with no fear.
Although, the better approach is to fork & use :git => path-to-your-fork, submit a pull request to the lib maintainer, share some love, and that stuff 😄

Edit: to explain why, if your changes are domain-related, you can always update your fork from upstream.