Adding config option for filtering models

[hughhhh]

Users now will be able to filter Model.config values when running the cli

$ dbt run --select config.materialized:view

# w/ config
{{ config(
  materialized = 'view',
  unique_key = ['column_a', 'column_b'],
  grants = {'select': ['reporter', 'analysts']},
  transient = true
) }}

[hughhhh]

resolves #187

[betodealmeida]

Looks like the tests are failing because you need to add config to the other models, even if it's an empty dict.

[betodealmeida]

Awesome!