Add ability to exclude tables in `schema` from being generated in `models.go`

[dhermes]

What do you want to change?

For example, in an auxiliary join for the widget table:

SELECT
  w.id,
  w.price,
  w.description,
  p.full_name AS owner_full_name
FROM
  widget AS w
INNER JOIN
  person AS p
ON
  p.id = w.owner;

we may never need a type in models.go for the person table.

What database engines need to be changed?

PostgreSQL, MySQL

What programming language backends need to be changed?

Go