Hook for printing additional instructions upon successful install

[shazow]

Use case in mind: urllib3 has optional features which are only enabled if some dependencies are installed. Would love to be able to print information about whether these features are available (and if not, which dependencies need to be installed).

Other use cases in mind: When installing PIL/pillow, it would traditionally print which file formats are supported (again, based on available dependencies).

Most distro package managers have a hook for this kind of stuff, would be handy for pip to have one as well.

Ideally the hook would execute some Python code so that the message can be dynamic (e.g. I'd like to be able to check whether I can import a module before I print the message).