Add post_uninstall function to formulae uninstallation

Useful for clean up commands after a formula has been uninstalled.

Example of formula ( [qt-mysql.rb](https://gist.github.com/dakcarto/7557064) ), where Qt's `qmake` auto-installs a built SQL driver plugin into `<qt.prefix>/plugins/sqldrivers` but `brew uninstall qt-mysql` will not remove it. The `post_uninstall` formula function handles the necessary cleanup by removing the dylib.

*Note:* I understand that Qt has a `--with-mysql` option, but the noted formula also works with bottled installs.

Is there another already means of achieving such a functionality?