Draft: Move Easyjet config to EasyConfig

I'd like to use the configuration we have in easyjet in some other projects. This creates a new package for this configuration.

There are a few things here, which don't have to work together (but can):

• A function that combines json files according to some rules I've used in a few places.
• A ConfigItem object that is essentially a read-only version of AthConfigFlags, but which supports json / yaml lists too.

To do:

• Explain all this better in the MR
• Add some README
• Add unit tests for the yaml file merger

Tagging @khoo

Closes easyjet/easyjet#121