Add offset support

[Serchinastico]

This branch contains several improvements:

• Add an offset support for easily handle headers in RecyclerViews with GridLayouts.
• Adds some useful shortcuts for selecting Directions.
• Simplify some internal logic in PositionAdapter
• Add a new OffsetSelector to apply dividers to items with an adapter index equal or higher than a specified position.
• Adds an eraser method to avoid painting a certain direction in DividerBuilder (instead of declaring the whole Divider as empty and declaring which sides should paint you can now do it the other way around).

[davideme]

👍