Configurable DateTime types

[narrowtux]

This PR adds the possibility to configure the DateTime module used to generate utc_now() datetimes and the types of all datetime fields in the user schema.

config.exs

config :coherence,  
  datetime_module: DateTime

user.ex

#...
  schema "users" do
    coherence_schema(datetime: :utc_datetime)
  end

[norbajunior]

hey @narrowtux thanks for your PR. I'll take a look as soon as possible.

[caironoleto]

@narrowtux thanks you for your PR. We'll take a look ASAP.

[caironoleto]

@narrowtux also, I think it's missing some tests, especially related to Schema.

[caironoleto]

why not use Config.datetime_module().utc_now() as a default param?

[narrowtux]

I believe that default parameters are decided at compile time - then all locked users would have the same locked_at date.

[narrowtux]

Maybe I got this wrong - this suggests that the expression is only evaluated when needed: https://elixir-lang.org/getting-started/modules-and-functions.html#default-arguments

[caironoleto]

yeah, look:

defmodule MarsRover do
  def time(foo \\ NaiveDateTime.utc_now()) do
    foo
  end
end

iex [1] > MarsRover.time
~N[2019-06-20 13:58:51.149503]
iex [2] > MarsRover.time
~N[2019-06-20 13:58:52.923541]
iex [3] >

Again, thank you! <3