Q: Overwritting a section partially In config/config.exs I have this:
  config :my_config_section, MyApp.Module1,
    a: "dfsafds",
    b: 1234,
    c: "aaaaaa",
    d: true

In config/dev.exs I want to overwritte only 1-2 keys :my_config_section. Without having to specify all the keys again. 
Is it possible?

A: As stated in the documentation for Mix.config/3

Keyword lists are always deep merged.

So yes, just overwrite keys you want, assuming you invoke import_config "dev.exs" after the generic config call.
