Added headless option to model constructor.

Not sure if this is worth merging, but it's an option to run "headless" (i.e. without creating any canvases, or triggering any draws). It's not super forgiving if you then try to use functions that presume you have canvas contexts to work with, but I think you're still able to use all of agentscript's modeling tricks and utilities without the sexy renderer (e.g. if you wanted to render to tiles, dom elements, etc.).