[pod plugins installed] List registered hooks (if any)

For example if `cocoapods-keys` is installed

```
$ bundle exec pod plugins installed

Installed CocoaPods Plugins:
    - cocoapods-keys    : 1.0.0 (post_install hook)
    - cocoapods-try     : 0.4.3
    - cocoapods-trunk   : 0.5.0
    - cocoapods-plugins : 0.4.0
```

```
$ bundle exec pod plugins installed --verbose

Installed CocoaPods Plugins:

cocoapods-keys
    - Version:  1.0.0
    - Hooks:
      - post_install
    - Homepage: https://github.com/cocoapods/cocoapods-keys
    - Summary:  CocoaPods Keys will store sensitive data in your Mac's keychain. Then on running
    pod install they will be installed into your app's source code via the Pods library.

cocoapods-try
    - Version:  0.4.3
    - Homepage: https://github.com/cocoapods/cocoapods-try
    - Summary:  CocoaPods plugin which allows to quickly try the demo project of a Pod.

cocoapods-trunk
    - Version:  0.5.0
    - Summary:  Interact with trunk.cocoapods.org

cocoapods-plugins
    - Version:  0.4.0
    - Homepage: https://github.com/cocoapods/cocoapods-plugins
    - Summary:  CocoaPods plugin which shows info about available CocoaPods plugins.
```
_(Note: to test this you need to add `cocoapods-keys`'s `master` to your `Gemfile`, as the official `0.9.5` gem does not yet register the associated plugin name — only `master` does it properly)_

/cc @orta 

To be fair, the method actually returns the list of the registered hook's *names*, not the registered hooks themselves, so we may still improve that doc ^^