Config: Allow default value for when environment variables not set

## I'm submitting a...

- [ ] Bug report
- [x] 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)