Allow for encrypted environment configuration

[dberesford]

Having encrypted secrets in the application environment is a requirement. The rough flow is as follows:

• app starts up, reads a 'secrets_encrypted' type setting
• if it's set, a certain number of env vars will be encrypted
• when the application config is created, these vars need to be decrypted (typically using KMS or something similar)
• the decrypted config is used in the application

Implementation notes:

• the config loading needs to be a singleton, i.e. this decryption should be done once and once only at startup
• the config loading needs to be asynchronous (not usually the case)

[davidmarkclements]

thoughts on https://www.npmjs.com/package/cryptex ?