Add Futures support to the component event loop

[hgzimmerman]

Description

** I'm submitting a feature request - please describe the behavior you want and the motivation.

There exists a fork of Yew called plaster that has support for, among other things, async calls being used in component event loops. With #684, it is possible to imagine people disabling the services feature and interacting with the browser using futures and web_sys.

I suggest that we attempt to port async support back to yew, with credit to @carlosdp of course.

The function at the following link appears to add support for async functions to the event loop: https://github.com/carlosdp/plaster/blob/master/src/html.rs#L135.

---

I have had trouble integrating web_sys constructs (and I'm assuming wasm_bindgen related things as well) directly into yew, resulting in indecipherable compiler errors when using cargo-web. Hopefully, the addition of this feature is pain-free. Considering that my prior failures to use web_sys might have been due to version mismatches, the work in #681 might make this less painful.

---

This issue takes inspiration from #636, but offers a more constrained and concrete set of actions to take in order to get part of that proposed featureset implemented.

[ctaggart]

it is possible to imagine people disabling the services feature and interacting with the browser using futures and web_sys

That is exactly what I would like to do.