Implement Paginator.Cursor in a more configurable way

[fuelen]

Use case: encode/decode DateTime in a shorter variant, like Unix timestamp or so.

Example:

a = DateTime.utc_now                              
#DateTime<2019-04-02 11:43:12.567049Z>

a |> :erlang.term_to_binary() |> Base.url_encode64()
"g3QAAAANZAAKX19zdHJ1Y3RfX2QAD0VsaXhpci5EYXRlVGltZWQACGNhbGVuZGFyZAATRWxpeGlyLkNhbGVuZGFyLklTT2QAA2RheWECZAAEaG91cmELZAALbWljcm9zZWNvbmRoAmIACKcJYQZkAAZtaW51dGVhK2QABW1vbnRoYQRkAAZzZWNvbmRhDGQACnN0ZF9vZmZzZXRhAGQACXRpbWVfem9uZW0AAAAHRXRjL1VUQ2QACnV0Y19vZmZzZXRhAGQABHllYXJiAAAH42QACXpvbmVfYWJicm0AAAADVVRD"

a |> DateTime.to_unix(:microsecond) |> :erlang.term_to_binary |> Base.url_encode64()
"g24HAA27XHaShQU="

As you can see, the latter is much shorter.