Allow nested span with inherited styles

[killercup]

It's not possible to have nested spans that inherit the parent's styles. One migth expect this however:

• span!(bg="red", [span!(fg="white", ["Error"]),]); shows the word "Error" as white text on red background.

What do you think? Is this a good idea or not?

---

This is because of this reset call:

https://github.com/killercup/output-rs/blob/abbe3f145954358457bf542af207784324e39290/src/components/span.rs#L110

It might make sense to call this here instead/additionally:

https://github.com/killercup/output-rs/blob/abbe3f145954358457bf542af207784324e39290/src/lib.rs#L75-L78