move shared files up to superclass

Addresses issue https://github.com/sethladd/stagehand/issues/20
Two thoughts:
- CDE will need to be able to add additional generators for our own use outside of this library. We won't want them to be too prescriptive
- even within stagehand, not all templates way end up wanting gitignore and LICENSE files

I'd suggest making a very simple subclass, `DefaultGenerator`, which creates these two files, and which the current generators in stagehand inherit from. It lets `Generator` be totally generic, and DefaultGenerator be more prescriptive.