Allow to customize the chef solo/client configuration

Right now Vagrant generates Chef solo/client configuration from a template, but the fields are fixed and it's imposible to add things that are not in the basic configuration, like report handlers for instance.

I was considering to add a new attribute to allow to use a configuration file provided by the user but I'm not sure if that's the best approach since I still need to add the paths that vagrant generates into the vm for the recipes.

Ideas and comments are really appreciated.