[Idea] Installation hooks for plugins

[soumyamahunt]

Currently install! command in Podfile only supports cocoapods as installation method, I think the scope of this command can be made broader by allowing plugins to specify installation hooks that user can then consume by specifying custom installation methods. If no registered hooks are found with the installation method then user will be prompted with error.

Although workaround is possible by using pre_install hook, this would make Podfile more readable in my opinion.