Preprocess YAML files as ERB.

This allows dynamic configuration files, and is in line
with how Rails preprocesses database.yml as ERB.

A primary use-case is configuration lines like:

   some_service:
     api_token: <%= ENV["SOME_SERVICE_API_TOKEN"] %>
README describes ERB preprocessing of YAML configuration files.
Merge pull request #2 from pda/erb-preprocessor

Preprocess YAML files as ERB.