enable passing in `pull-request` title & body

Emulates the interface of `git commit`:

  $ hub pull-request -m "This is my title"
  $ hub pull-request -F /path/to/message.txt
  $ hub pull-request -F -

The old way of passing in just title is deprecated

  $ hub pull-request "Deprecated title"

Fixes #183
simplify assertion of supplied API params