per-user configuration file for gle

It would be useful to have a configuration file that can be used to set options automatically. Such as always setting an environment variable or adding an additional docker volume.

Parse a config file like the one below from $HOME/.gle/emulator.yml or $GLE_CONFIG if set.

# ~/.gle/emulator.yml
emulator:
  # variables to set for all jobs
  variables: 
    SOME_VARIABLE: some-value

  # things to set only for docker jobs
  docker:
    variables:
      ENABLE_COMPILER_CACHE: /caches/ccache
    volumes:
      - /home/inb/ssd/caches:/caches:rw

  # things to set only for non-docker jobs
  local:
    variables:
      ANOTHER_VAR: value