Support function decorators for throttling

[cabol]

By means of Decorators Lib, we could implement something like:

defmodule SomeModule do
  @moduledoc false

  @decorator throttle(counter: :send_request, max_rate: 100)
  def send_request(req) do
    # some logic ...
  end
end

This will be another option for throttling (aside from the one mentioned in issue #4)