in-model configs, show config options in prologue

[drewbanin]

With this branch, configurations can be specified directly inside of models. The configs work exactly the same as configs inside of the dbt_project.yml file.

An in-model-config looks like this:

{{ config(materialized="incremental", sql_where="id > (select max(id) from {{this}})") }}
-- or you can use python dict syntax:
{{ config({"materialized:" "incremental", "sql_where" : "id > (select max(id) from {{this}})"}) }}
-- you can only call config() once in a model, so don't do both!

select * from public.users

The config options are injected into the existing configs (more on this below) and the config(...) function call is replaced with a SQL comment containing the injected config options, eg:

-- Config specified in model: {'materialized': 'incremental', 'sql_where': 'id > (select max(id) from {{this}})'}

select * from public.users

If the model is in your package, then config resolution works in the following order (later configs overwrite previous configs):

1. model-defaults (dbt_project.yml)
2. in-model configs (your_model.sql)
3. models (dbt_project.yml)