Add helper methods for setting pre-and post-hooks

[drewbanin]

Feature

Feature description

See also #558

Presently, in-model hook definitions need to be set in a {{ config() }} block. This isn't the most natural place for long-form SQL. Instead, dbt should provide a set of helper macros for setting pre- and post- hooks. For example:

{{ config(materialized='table') }}

{{ post_hook("grant select on {{ this }}" to db_reader") }}
{{ post_hook(some_complex_macro()} }}

This will also be a nice interface for inclusion in higher-level macros.

Who will this benefit?

Users with many model-specific hooks.

[jakebiesinger]

IMHO, variables would probably handle this more easily and without work from you:

{% set post_hook %}
  {% for role in ['db_reader', 'db_writer'] %}
    grant select on {{ this }} to {{ role }};
  {% endfor %}
{% endset %}

{{ config({ 'materialized': 'table', 'post-hook': post_hook}) }}

Probably deserves an update to your documentation though, since folks may not know about multi-line set blocks.