- use_labels flag on select() wont auto-create labels for literal text

- use_labels flag on select() wont auto-create labels for literal text
column elements, since we can make no assumptions about the text. to
create labels for literal columns, you can say "somecol AS somelabel",
or use literal_column("somecol").label("somelabel")
- quoting wont occur for literal columns when they are "proxied" into the
column collection for their selectable (is_literal flag is propigated)