Built-in codec for DateTime.t().
Encodes DateTime structs to ISO 8601 / RFC 3339 strings
(e.g. "2012-04-23T18:25:43.511Z") and decodes them back.
Handles :json and :binary_string formats (returning a binary string) and
:string format (returning a charlist).
Registration
Registered automatically when the :spectral application starts. To use a
different codec for DateTime.t(), set it in config :spectra, :codecs;
user-configured codecs take precedence over the built-ins.