Feature/simple hooks

[drewbanin]

Implements the simple version of hooks. In your dbt_project.yml file:

models:
  'Test DBT':
    pre-hook: "insert into {{ ref('audit') }} (model, timestamp, status) values ('{{this.table}}', getdate(), 'started')"
    post-hook:
       - "insert into {{ ref('audit') }} (model, timestamp, status) values ('{{this.table}}', getdate(), 'completed')"
       - "grant select on {{this}} to some_user"

There are a couple of new things here:

• pre hooks run before a model is executed
• post hooks run after a model is executed
• hooks can be defined as either a string, or a list of strings
• these hooks are a part of the transaction which materializes a model. If the transaction fails, the hooks will not be run.
• the {{ this }} var has been extended such that you can use
  • {{this}} to interpolate "schema"."table"
  • {{this.schema}} to interpolate "schema"
  • {{this.table}} to interpolate "table"
• hooks are appended to child configs. A hook defined directly beneath the project config will be applied to every model in that project. Hooks defined more specifically will be augmented, not overwritten.
• hooks are not applied to ephemeral models

cc @jthandy