Added support for editing new posts with --edit

I notice there was once support for automagically launching an editor on new posts. This seems to be a super obvious feature, and others (e.g., https://github.com/cjfrisz/frog-plus) add it via shell scripts on top of Frog, so I'm guessing it was removed because it's hard to make it work with all the unique combinations of editors and environments out there ...

But I didn't want to resort to a bash script parsing the output of Frog, so I hacked up something I think should work for most people.

I welcome comments and criticisms.