Add counters support – increments and decrements by a given amount

[cabol]

Create a callback update_counter in Nebulex.Cache, in order to handle both scenarios, increments and decrements, depending on the incr value, which can me positive or negative. In case incr is not a valid integer value, it should raise an ArgumentError exception.

@callback update_counter(key, incr :: integer, opts) :: value | no_return

By default incr might be 1.