Allow to fully customize separator widget with a builder

[mauriziopinotti]

I added a new parameter to ResizableWidget: separatorBuilder. This allows consumers to fully customize the separator using a builder.

Please notice this patch is built on top of #10.

This lets you fully customize the separator like this:

    return ResizableWidget(
      isHorizontalSeparator: true,
      separatorSize: 16,
      separatorBuilder: (info, controller) => DefaultSeparatorWidget(
        info: info,
        controller: controller,
      ),
      children: [
        // ...
      ],
    );

[panval]

hi! I'm new to Flutter - or to Coding at all.
I'd like to change the separator to a "classic" two liner

I managed to get the separatorBuilder Option however, I don't know how to use it / or how to build the class for it (DefaultSeparatorWidget)