Read from stdin

If the user passes `-` as the filename, read from stdin instead.

Alternatives that do not work:

- Use isatty(3) to figure out whether the user is piping in and, if so
  read from the pipe; otherwise open the editor. This has weird behavior
  when used from e.g. desktop environments, so bail on that.
- Try to read from stdin and, if that produces the empty buffer, open
  the editor. The program blocks until the user presses ^D in this case,
  so skip that idea.
Add a NEWS file