Added the possibility to have multiple slivers as child

[UnderKoen]

With this pr you can have multiple slivers as child what would make it possible for instance to have multiple sticky headers as child, or 2 lists.

Also solves #62

[prakashjaganathan]

Multiple sticky headers works fine... but the issue is, click action not working for bottom placed childs... If I scroll up the same child to top, then click action is working fine... can anyone fix this issue?

[UnderKoen]

I will look into it

[prakashjaganathan]

I will look into it

Can you please check those buttons click inside child sliver list... because, it's working fine on headers and only child list item actions are not working...

[cgestes]

Having issue with hitTesting child when having two level of StickyHeaders. Otherwise I love the feature :)

Managed to make nested StickyHeader work (including hitTesting) by using MultiSliver instead of the slivers argument for StickyHeader.
StickyHeader(sliver: MultiSliver(children: [..., StickyHeader(...)])

:D

[letsar]

Thanks for the PR, but I fixed a bug, and know we can create nested sliver sticky header with MultiSliver. I think it's the best approach.