Proposal: open $EDITOR when creating a new issue

This would work basically the same way `git commit` does when a commit message isn't provided on the command line. The first line in the resulting message will be used as the issue title, any subsequent non-comment lines will be used as the issue description.

I imagine there might be some strong feelings about this feature one way or the other, so please feel free to decline this pull request.  I also imagine there might be some strong feelings about this particular implementation (*particularly given that it doesn't actually work correctly right now*).  I'd be happy to learn about better approaches to solving this problem.

One way in which this implementation differs significantly from that of `git commit` is that this will create a random temp file to use, while `git commit` uses a known file location: `.git/COMMIT_EDITMSG` in whatever repo you're committing to.

Thoughts?