Allow setting fragments for styles

[Kobzol]

It would be nice to have the ability to "scope" styles to specific fragments, or change their properties for a given set of fragments.

[spirali]

I was thinking about it and at least partially it can be done as follows:

b1 = slide.box(step="1,3,5")
b1.set_style(...)

b2 = b1.overlay(step="2,4")
b2.set_style(...)```

[Kobzol]

Good idea. It wouldn't solve all use-cases, but could bt useful for some.