Implement IntoStyle for (str, Option)

[luxalpa]

This is my attempt at allowing (str, Option<T>) in IntoStyles for #3568 . As I am not very familiar with the code base there might be things I am misunderstanding.

I also wanted to fix the case for a ReactiveFunction<Output = Option<T>> however the way I see it it might require a refactor, due to possible overlapping impls.

Also, I feel like in general something like trait IntoStyleValue could be useful.

[gbj]

Thanks for your work! Having looked at it a little today, I agree with your idea that fixing it for the reactive case requires larger refactoring. I've made a small start and will try to take a crack at it this weekend -- I'm going to leave this open for now until I see what needs to happen.

[gbj]

Okay, I think I was able to do a more general implementation of this in #3618 — I made the PR against the 0.8 branch as it includes some semver-breaking changes I think. (Good idea to use a helper trait.)

[luxalpa]

Perfect, thank you!