Support persist docs on Postgres and Redshift: Relations and columns

[drewbanin]

Child issue of #1573

Notes:

• We can render multiline comments using this string escaping syntax

To do:

• Support persisting docs for relations (tables and views)
• Support persisting docs for columns in tables and views

Syntax:

comment on table {{ relation }} is $__dbt_comment__${{ comment }}$__dbt_comment__$;

comment on column {{ relation }}.{{ column }} is $__dbt_comment__${{ comment }}$__dbt_comment__$

If the project specifies a column that does not exist in the destination table, the comment on column statement may fail. This is the desired behavior.