[Idea] Installation hooks for plugins

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.