Patches the specified application's Mix configs upon project startup.

Usage

Set the following in your config/prod.exs:

config :confix, :patch,
  my_project: MyProject.Web.Endpoint, # patches specific key in specific application
  project_2: [], # patches all keys in specific application
  project_3: [:adapter, :url] # patches only given nested key in specific application

That's it. The configuration for specific apps and keys will be parsed using Confix.parse_deep/1 and persisted in the Mix config. So all system tuples will be replaced with actual environment variable values, with optional casting and defaults as specified in the Confix module docs.