parser: Support columns from subselect statements

[maxhawkins]

Fixes #467

[peteole]

Cool that it works, however there is still an issue when using joins in a subselect statement.
Take this example:

SELECT r.*
FROM (
        project_saves
        JOIN projects
        ON project_saves.project_id = projects.id
    ) r;

This fails with the message

# package sqlc
sql-queries/projects.sql:159:1: empty edit contents

sqlc seems to believe that thesubselect r has no columns since when selecting a field of r the error message is "column "id" does not exist". Is there an easy way to fix this?