[UnusedConfigurationOptions] config/3 not being properly checked

[pbrudnick]

Bug Description

The rule only use a logic for analyzing config/2.

To Reproduce

Setting a config/3 in the config.exs, like a common Phoenix app does:

config :myapp, MyAppWeb.Endpoint,
  environment: Mix.env(),

Errors out:

:0 - In module : Configuration option MyAppWeb.Endpoint (MIX_ENV=dev) is not used anywhere in the code

Expected Behavior

Check the option for the context (2nd argument)