Added PubSub package and sample

[derNiklaas]

closes #116

uses: https://www.npmjs.com/package/twitch-pubsub-client

Note: I only tested the onRedemption function. I don't know if the others work, but they should work.

[hlxid]

Code looks pretty good so far 👍.
However I would like the package to be renamed to nodecg-io-twitch-pubsub (and the service to twitch-pubsub) to make it clear that is for twitch as there may be other pubsub services that we may want to add in the future.
(chat will also be renamed to nodecg-io-twitch-chat for clarity but that can be done in another pull request and later).

Failing CI because of missing express-transform-bare-module-specifiers@1.0.3 is fine. The older versions were removed from the npm registry and 1.0.4 is available. The package-lock.json still uses 1.0.3 which results in the error. Will be fixed soon anyway as I want to bump all dependencies soon (maybe tomorrow if I have some time?).

[derNiklaas]

The last commit ( fa62cae ) should've satisfy all requested changes.

[hlxid]

Thanks!