address_gen.js does not generate internal addresses

[address_gen.js](https://github.com/ava-labs/avax-js-cli-tools/blob/master/address_gen.js) generates external addresses only by default:

``` javascript
const AVAX_ACCOUNT_PATH = `m/44'/9000'/0'`;
let derivationPath = `${AVAX_ACCOUNT_PATH}/0/${i}`;
```

It would be nice to have option to also generate internal addresses (used when generating "spare change" in a transaction):

``` javascript
const AVAX_ACCOUNT_PATH = `m/44'/9000'/0'`;
let derivationPath = `${AVAX_ACCOUNT_PATH}/1/${i}`;
```

For some background info on the difference on external vs internal addresses, see:
https://medium.com/@harshagoli/hd-wallets-explained-from-high-level-to-nuts-and-bolts-9a41545f5b0

Start with address_gen.js and trace how it builds the derivation path and accepts command-line options. Add a way to generate internal addresses using the /1/i branch while preserving the current external /0/i default. Done means both address types can be generated and the existing external behavior remains unchanged.

Written by the indexing model from the issue text.