Ability to add external Lua dependencies for custom policies

Currently, when developing custom policies, there is no easy way to include or use external Lua libraries.

The request is to provide a way to easily add Lua dependencies to APIcast. This will enable a lot more flexibility for users building custom policies as it will allow making use of external dependencies not included in the productized image.

Demo: https://github.com/3scale-demos/apicast-json-to-xml (small policy that transforms JSON response to XML. It explains how to use external dependencies.)