Support schema export, similar to "pg_dump --schema-only"

Do you want to request a *feature* or report a *bug*?
-----------------------------------------------------
Feature.

What is the current behavior?
-----------------------------
N/A

What is the expected behavior?
------------------------------
Expected behavior is that the schema of a heroku instance can be exported similar to the way that "pg_dump --schema-only" works right now. Since the database in use is in a private space, it's not possible to connect directly to a private-tier database without having additional IP ranges exposed (You'd need to connect directly to the database to use the `pg_dump` cli tool).

Ideally:

```
heroku pg:schema:export -o schema.sql -a my-app
```

Workaround right now is to stand up a local postgres and run heroku pg:pull and exclude table data for all tables, but that requires you to maintain the list of tables/etc.