Support string interpolation in configuration file

[dshafer]

This PR will allow us to inject configuration through environment variables referenced in config/restforce.rb, e.g.:

host: "<%= ENV['SF_HOST'] %>"
username: "<%= ENV['SF_USERNAME'] %>"
password: "<%= ENV['SF_PASSWORD'] %>"
security_token: "<%= ENV['SF_SECURITY_TOKEN'] %>"
client_id: "<%= ENV['SF_CLIENT_ID'] %>"
client_secret: "<%= ENV['SF_CLIENT_SECRET'] %>"

This is required for containerizing and automating build/test for Federation Manager.

[ij]

Thanks for the pull request.

[ij]

Fyi, I assigned the tag v0.1.53 after merging.