Don't error when previewing buffer without file

[xiongtx]

Fixes #200.

[jrblevin]

Thanks for this. It looks OK to me, but user-error is not defined on Emacs 24.1 and 24.2, so the CI tests are failing. Can you please fix that? Two possibilities are:

1. Maybe just use error instead?
2. Add a markdown-user-error defalias that falls back to error when user-error is not bound. See the "Compatibility" section of the code for some examples. Something like this should work:

(eval-and-compile
  (defalias 'markdown-user-error
    (if (fboundp 'user-error)
        'user-error
      'error)))

Update: I see you added a second commit now. My comments above refer to 96f9d4f.

[xiongtx]

Yeah, I tried to use skip-unless but that caused the Emacs 24.3 test to fail as well.

In the end I just went with error. At some point support for Emacs <24.3 should probably be dropped.