Config: Allow default value for when environment variables not set

[kael-shipman]

I'm submitting a...

• Bug report
• Feature request
• Question

Current behavior

You can specify environment variables for config, but if the environment variable is not set, you can't provide a default (fallback) value.

Expected behavior

Should be able to provide { "ENV": "MY_ENV_VAR", "default": "some-value" } as a value, and when MY_ENV_VAR is not set, some-value should be used.

Environment

db-migrate version: 0.11.13
db-migrate driver with versions: mysql v2.2.0

Additional information:
- Node version: v17.3.0
- Platform: Linux (Ubuntu 20.04.3)