Add schema comments to models.go

If I attach a comment to a table or column in the schema it should appear as a comment in the generated Go code.

```sql
CREATE TABLE foo ( bar INT );
COMMENT ON TABLE foo IS 'Foo is a table';
COMMENT ON COLUMN foo.bar IS 'Bar is a column';
```

```go
// Foo is a table
type Foo struct {
  // Bar is a column
  Bar int
}
```