work in progress

[drewbanin]

This fixes #373. This branch is functionally about what i think is necessary here, but there's a decent amount of cleanup to do before we can merge this.

@cmcarthur can you think of any way to reliably discover refs without injecting ourselves into the lexer? I think once we get any higher up the chain than that, we'll lose the ability to traverse conditional branches.

This does not work if the thing passed to ref is a variable, eg:

{% models = ['model_1', 'model_2'] %}

{% for model in models %}
  select * from {{ ref(model) }} union all
{% endfor %}
....