Add a version of decodeFile with support for a !include directive

This adds support for includes in YAML files.  Our use case is to have credentials and other config values in one YAML file during development, but include the credentials from elsewhere on production.

For the used syntax, @soenkehahn did some research on what others use and suggested `!include <filename>` (see https://github.com/zalora/yaml/issues/1#issuecomment-61438368).