Post-config hook to make template adjustments

This allows upstream libs to let cfn-config prompt the user for parameters, and then make adjustments to the template body based on the answers to the prompts. This need comes out of [discussion about watchbot and permissions](https://github.com/mapbox/watchbot/issues/40), which is currently blocking https://github.com/mapbox/mapbox-com-publish/pull/47. 

Specifically, watchbot needs to know what "role" the user has assigned the stack (development, staging, production) before it can add permission to access the proper AWS resources for that role.

cc @willwhite @jfirebaugh