save and reuse pull request message if creating it failed
improve preserving message from failed pull request

 - If text editor aborts, message is discarded.
 - If the API request to create the PR fails, message is kept.
 - If the API request succeeds, message is discarded.
 - When kept message is reused for subsequent PR, only its title and
   body get inserted. The PR instructions and changelog still get
   regenerated.

Fixes #178
avoid hard breaks in pull requests authored in vim

Hard-wrapping at 80 chars looks great for Markdown and commits, but
since pull request bodies are rendered with GitHub-flavored Markdown, we
want to avoid line breaks because they get preserved and look like crap
on the web.

Fixes #318
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
Merge `pull-request` changes