redshift comments.sql

-- pseudocode:

```
{% set table_comment = "here is a comment about my table" %}
{% set column_comments = {
  "column_1": "column 1 comment is...",
  "column_2": "column 2 comment is...",
}
%}

{{
  config({
    "post-hook": add_comments(this, table_comment, column_comments)
  })
}}

...model sql...
```

This would require a macro like:
```
{% macro add_comments(this, table_comment, column_comments %}

COMMENT ON TABLE {{ this }} IS '{{ table_comment | replace("'", "") }}';
{% for column, column_comment in column_comments.items() %}
  COMMENT ON COLUMN {{ this }}.{{ column }} IS '{{ column_comment | replace("'", "") }}'";
{% endfor %}