Drop Protocol - `to_number` / `__int__`

[jg-rp]

In addition to to_liquid_value (allows drops to behave like a Liquid primitive value inside some expressions), Ruby Liquid uses to_number to allow drops to work with filters that expect numbers as arguments.

The same effect can be achieved in Python Liquid using an __int__ and/or __float__ method.

Python Liquid's documentation for drop objects needs to be expanded to cover this use case.