add --passive option for backgrounding

It appears rerun doesn't like to be backgrounded:

    $ bundle exec rerun "bundle exec ruby app.rb" &
    [rerun] App launched
    stty: standard input: Inappropriate ioctl for device
    stty: standard input: Inappropriate ioctl for device
    stty: standard input: Inappropriate ioctl for device
    stty: standard input: Inappropriate ioctl for device
    <ad infinitum>

Presumably that's happening within the `key_pressed` method when attempting to read STDIN.

Thus this new `--passive` option* disables the on-the-fly commands entirely.


\* not sure whether `-b` is a good short variant, but `-p` was taken