Add basic features to download and use precompiled NIFs

[philss]

This is based in the work made for PR https://github.com/rusterlium/rustler/pull/419

The usage is similar to Rustler. For example, the HTML5ever native module will look like this:

defmodule Html5ever.Native do
  @moduledoc false
  require Logger

  mix_config = Mix.Project.config()
  version = mix_config[:version]
  github_url = mix_config[:package][:links]["GitHub"]

  rustler_opts = [otp_app: :html5ever, crate: "html5ever_nif", mode: :release]
  env_config = Application.get_env(rustler_opts[:otp_app], Html5ever, [])

  # This module will be replaced by the NIF module after
  # loaded. It throws an error in case the NIF can't be loaded.

  if System.get_env("HTML5EVER_BUILD") in ["1", "true"] or env_config[:build_from_source] do
    use Rustler, rustler_opts
  else
    use RustlerPrecompiled,
      otp_app: :html5ever,
      crate: "html5ever_nif",
      base_url: "#{github_url}/releases/download/v#{version}",
      version: version
  end

  def parse_sync(_binary), do: err()
  def parse_dirty(_binary), do: err()
  def flat_parse_sync(_binary), do: err()
  def flat_parse_dirty(_binary), do: err()

  defp err, do: :erlang.nif_error(:nif_not_loaded)
end