Make it possible to implement Cookiecutter plugins like:

• @pydanny's secret key plugin and similar
• a basic input validation plugin that can be extended or swapped out (see Template can create python modules with invalid names audreyfeldroy/cookiecutter-pypackage#6)

This issue is dependent on #29 / #48.

I wonder if there would be a way to implement the plugin system such that #55 would be possible as a plugin. Not sure.