internal/dinosql: Add COMMENT ON support

[kyleconroy]

Comments are supported for SCHEMA, TABLE, COLUMN, and TYPE.

Fixes #187

[maxhawkins]

Related to #193:

This breaks if one of your comments is on a view.

CREATE TABLE users (name TEXT);

CREATE VIEW harrys AS
  SELECT name
  FROM users
  WHERE name ILIKE '%harry%';

COMMENT ON harrys.name IS 'Harry's name';

relation "harrys" does not exist