Support default values in environment variable interpolation

[AngrySoundTech]

It would be nice if were were able to set default values in config variables, to be able to be overrided.

For this, I'd propose something similar to Shell Parameter Exansion, so for example

database:
  host: ${CFG_DATABASE_HOST:-mariadb}
  database: ${CFG_DATABASE_DATABASE:-minecraft}
  password: ${CFG_DATABASE_PASSWORD}

with env vars set of

CFG_DATABASE_HOST=mysql
CFG_DATABASE_PASSWORD=letmein

would result in

database:
  host: mysql
  database: minecraft
  password: letmein

[itzg]

That's a great suggestion. I'll get that queued up.