Pass --prompt default to ruby and jruby

Without it (or --inf-ruby-mode), IRB in Emacs on Windows defaults to
no prompt at all. The problem isn't limited to IRB, so we probably
should recommend fixing it via .irbrc instead, like here:

http://stackoverflow.com/questions/11221345/rails-console-runs-without-prompt
https://github.com/eschulte/rinari/issues/42#issuecomment-10626120

Haven't tested it with rubinius. I'm not modifying the yarv entry
because I don't believe anyone's using it (also, having different
versions of Ruby installed on Windows should be very uncommon).
inf-ruby-load-file is not defined
inf-ruby-output-filter: Ignore empty strings

* They happen with comint-process-echoes t.
* Only modify the local comint-output-filter-functions value.
inf-ruby-fix-completions-on-windows: Fix free variable compilation warning
Set comint-process-echoes on Windows

Close #8