Adding support for enums

[kai zhu]

hi @peter, appreciate your perspective for embedded-device use-case.
however, as @isiah mentions, there are existing javascript
design-patterns using dicts/lists that are superior to enums.  the
primary reason (which was implied) is because they can be easily
stored/shared/parsed as json config-settings, e.g.:

```json
{
    "DEVICE_LIST": [
        {
            "TYPE": "ARDUINO",
            "ADFlag": {
                "LE_LIMITED_DISCOVERABLE_MODE": 1,
                "LE_GENERAL_DISCOVERABLE_MODE": 2,
                "LE_GENERAL_DISCOVERABLE_MODE_ARDUINO": 3,
                "NO_BR_EDR": 4
            },
            "ADType": {
                "INCOMPLETE_UUID16_LIST": 2,
                "COMPLETE_UUID16_LIST": 3,
                "INCOMPLETE_UUID128_LIST": 6,
                "COMPLETE_UUID128_LIST": 7
            }
        },
        {
            "TYPE": "RASPBERRY_PI",
            "ADFlag": {
                "LE_LIMITED_DISCOVERABLE_MODE": 1,
                "LE_GENERAL_DISCOVERABLE_MODE": 2,
                "NO_BR_EDR": 4
            },
            "ADType": {
                "INCOMPLETE_UUID16_LIST": 2,
                "COMPLETE_UUID16_LIST": 3,
                "INCOMPLETE_UUID128_LIST": 6,
                "COMPLETE_UUID128_LIST": 7
            }
        }
    ]
}
```

you can also save/update these configs to a github-repo (with
encryption like this real-world example [1]), and perhaps have your
iot device poll it every 5 minutes for effortless config-updates in
near-realtime.

-kai

[1] encrypted travis-ci-config-settings hosted on github
https://github.com/kaizhu256/node-utility2/blob/gh-pages/CRYPTO_AES_SH_ENCRYPTED_kaizhu256