Use runtime config for assets

[szajbus]

Summary of the PR:

• Use runtime config instead of compile time

The main use case for providing runtime config is to override local_directory option when using Elixir releases:

# config/runtime.exs
config :absinthe_plug, Absinthe.Plug.GraphiQL,
    assets: [
      source: :smart,
      local_url_path: "/absinthe_graphiql",
      local_directory: Path.join(Application.app_dir(:my_app), "priv/static/absinthe_graphiql")
    ]

The above is a method to use patched version of @absinthe/socket dependency which fixes the missing headers bug (see absinthe-graphql/absinthe-socket#67)

As far as I know this cannot be achieved using compile-time configuration, because Application.app_dir(:my_app) is not available at that time. Besides, compile-time configuration didn't work anyway - it was inducing a compilation error.

• Remove assets option from plug's init/1 type specification, because it was not used there.