Libify

Tries to hit the issues raised by #9.

- Exposes more CFN workflow as exports from `index.js`. These are CRUD functions on a stack plus `stackInfo`. Added an optimist script in `/bin/` for each function.

- Reads configuration files to provide prompting defaults. During an update operation, reads stack parameters from AWS to give prompt defaults.

- (untested) Exposes [a property that higher-level libs should be able to override](https://github.com/mapbox/cfn-config/blob/other-defaults/index.js#L29-L38) to provide their own default values / functions to be used by inquirer. 

cc @willwhite