Added "pre-autoload-dump" event.

This event is fired before the autoload file is generated, for either an install or update command.

To give some insight into a potential use case. In the Laravel framework, we compile common classes using Michael Dowling's class compiler (https://github.com/mtdowling/ClassPreloader). We do this POST autoload generation. It would be helpful for us to be able to clear out the old file PRE autoload generation.

Tests pass and documentation updated as well.