Allow some components to be customized

[orevial]

Hi @bazookon, as mentioned in issue #55 I'm proposing a PR for adding more customization to this package. I've focused on three parts:

1. Allowing to override the default tab top (the handlebar)
2. Allowing to override the default tab bottom (Giphy's credits)
3. Allowing to override the search part component. In this case the logic part of the search is still made by the original component, only the UI part is overriden.

Note that these overrides are all optional as we default to the default widgets you already developed. Optional params have been added to the doc as well.

Once last thing: I've added the ability to read the shape of the bottom sheet from the theme if any (Theme.of(context).bottomSheetTheme.shape), otherwise we default to the shape you chose before.

I also updated most dependencies to last versions, hope that's okay

Closes #55