Improve serverless.yml support

[artoliukkonen]

The tool automatically loads serverless.yml config, but it's pretty basic (plain YAML parse). If serverless.yml contains e.g. ${file} references the tool will not work as parser will not import those references.

Since the license allows it might be a good idea to copy&paste&refactor Variables class from Serverless Framework https://github.com/serverless/serverless/blob/master/lib/classes/Variables.js#L45 .

[BenEllerby]

Seems a good idea @artoliukkonen - is this something you want to take? Otherwise, I'll add it to our backlog :)