make `default` option work for specifying a config-path

`yargs.config("c").describe("c", "Config file path").default("c","config.json")` will now work as expected.

We should probably make this a `2.x.x` release, because it changes the order of `['baz', 'bar']` to `['bar', 'baz']` (in reality, this shouldn't effect anyone unless they're writing really wonky code).