Add separate configs for specific environments

[vitorbal]

As we scale out the usage of eslint-config-zapier, we will probably need config "subsets" (or supersets) for specific environments.
For example, a Zapier project using node would benefit from some additional node-specific rules and plugins. Or maybe a project is not interested in using flowtype-specific rules.

Creating additional configs is as easy as exporting a new config from a separate file:
http://eslint.org/docs/developer-guide/shareable-configs#sharing-multiple-configs

Checklist

• prettier subset
• node subset