Ingest data files into dynamic configs

[briandominick]

Similarly to how standard parse actions are able to pass data from multiple ingested files into the template used in the action, config files should also be able to accept data objects when they are templatized with Liquid scripting.

A command like the following should enable multi-file ingest of data objects:

bundle exec liquidoc -c _configs/build.yml -d data/products.yml,data/docsets.yml

This would create two data objects (products and docsets) that can then be referenced as Liquid variables from anywhere within build.yml.

This issue relates to #83, #84, and #40.