Set default S3 options for Uploader: signed & default_expiry_time

[rlopzc]

This is not a bug report, more like a feature request.

It would be nice to set some default options to an uploader. i.e. signed: true, expires_in: 1 hour

defmodule Okani.Uploaders.PatientImage do
  use Waffle.Definition
  use Waffle.Ecto.Definition

  @default_signed true
  @default_expiry_time 1 * 60 * 60

Right now I'm writing a custom url function with those defaults.

I can work on it when I have time and submit a PR. Other ppl feel free to take this one :)

Thanks for your work on the lib!