We split `send_invoice` offers inoo offerout (for want of a better name).
This simplifies the API.
Also took the opportunity to move the `vendor` tag to immediately
follow `description` (our tests use arguments by keywords, so no
change there).
Suggested-by: shesek
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Rusty Russell