feat(config): Start warning on unknown flags or config (under `loglevel=warn`)

[darcyclarke]

### Tasks
- [ ] https://github.com/npm/nopt/pull/179
- [ ] https://github.com/npm/nopt/issues/41
- [ ] https://github.com/npm/statusboard/issues/932

[wraithgar]

we should also warn when npm is "helpfully" allowing for abbreviated versions of any config item, with the goal of eventual removal in a future major

$ npm config ls -a -- --bef=2010-10-10|grep before
$ npm config ls -a --bef=2010-10-10|grep before
before = "2010-10-10T00:00:00.000Z"

[wraithgar]

#934