Allow globals and functions to be set in options

Not sure why these options are ignored right now...? But [my stylus broccoli filter][1] relies on the options hash as the only way to control rendering, so these options would be quite helpful (I just had someone on IRC quick-patch his local copy of stylus with this fix, but I'd certainly rather avoid that in the future)

[1]: https://github.com/gabrielgrant/broccoli-stylus-single