st3m/mode: allow displaying on screen when in repl mode

Tested by doing the following in REPL:

    >>> import hardware
    >>> ctx = hardware.get_ctx()
    >>> ctx.rgb(1, 0, 0)
    <ctx_type>
    >>> ctx.rectangle(-120, -120, 240, 240)
    <ctx_type>
    >>> ctx.fill()
    <ctx_type>
    >>> hardware.display_update(ctx)

Fixes #25.