Add 6x9 Paper Size

[Jay Anderson]

6 x 9 in. isn't currently available. I haven't found a standard name
for this size. Lulu calls it 'US Trade',
http://en.wikipedia.org/wiki/Book_size calls it 'octavo'. In any case
it seems to be a fairly common size that would be useful to add (and
custom sizes are clunky to add and maintain - is there a better way
besides modifying paper.scm?). My recommendation to add to paper.scm:

("6x9" . (cons (* 6 in) (* 9 in)))

Thanks.

-----Jay