Node 12 support

[kryops]

Hi there!

I tried upgrading my application to Node 12, and the following compile errors were thrown:

• v8::Handle does not exist any more
• args[i]->NumberValue() does not work any more

I don't really know C++, but I managed to get it working with the following changes:

https://github.com/kryops/node-usb-detection/commit/20ebc8bc2ade4319087d6476cf9f556c1c582b84

Could you please check if they make sense and don't break anything else (especially older versions of Node), and upgrade this package accordingly?

Thanks!

See also:
bcoin-org/bcrypto#7
nodejs/node#23122

[MadLittleMods]

@kryops Mind creating a pull request so we can add comments to the code itself that will be self contained in this project?