Allow nested span with inherited styles

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

Read src/components/span.rs around line 110 and src/lib.rs around lines 75-78 to understand where reset behavior is applied. Verify the nested span example with a red parent and white child, and consider the change complete when the child text renders white while retaining the red background.

Written by the indexing model from the issue text.