Tab completion hook for the %%opts cell magic.

def option_completer(cls, k,v):
        "Tab completion hook for the %%opts cell magic."
        line = v.text_until_cursor
        completions = cls.setup_completer()
        compositor_defs = {el.group:el.output_type.__name__
                           for el in Compositor.definitions if el.group}
        return cls.line_completer(line, completions, compositor_defs)