[bidi demo] Expose global direction, rtlMoveVisually and per-line direction keys.

demo: https://rawgit.com/cben/CodeMirror/37a71a50360a7379e3b1ac55768b96ae8c357feb/demo/bidi.html

Wasn't sure if I should use `setOption("direction"` or `doc.setDirection(`.

Overall direction branch is great improvement!  Will send feedback on mailing list after more testing.