Defuse time bomb on EditLine Readline wrapper

If you are using the basic OS X Readline impl, and you Pry.auto_resize!,
you'll find that their Readline.set_screen_size = ___ segfaults.

Instead of letting this happen, we'll tell the user about it and point
them to the fix (of using GNU Readline).
Experimental logic for Readline.output

The symptom of not having this patch is to:

    foreman start -f spec/Procfile

You'll notice you cannot see the stdout. This is a weird glitch, but the
following does fix it.

What do you think?
Guard against windows? + style homogenization
Adding output.flush per @ConradIrwin
Merge pull request #846 from pry/stdout-fix

Output visibility, e.g. for foreman-invoked processes