Allow or facilitate spawning actors on the UI thread

[kamalmarhubi]

I got here via the talk at Chrome Dev Summit 2018, where it was suggested that sometimes leaving things on the main thread is a good idea to avoid copying overhead. It'd be useful to include a built-in strategy, or at least a recipe in the readme for doing that.

(I haven't looked at the source enough to know if there's a reason this isn't possible.)