Add support for Multi-Month Subscription Gifting

[trezy]

Acceptance Criteria

As a developer, subgift and anonsubgift commands support the --months flag and return the msg-params-gift-months tag

Notes

• Twitch Announcement: Updates to PubSub Messages and IRC for Multi-Month Subscription Gifting

[trezy]

Support has been added; follow-ups required:

• Update docs site
• Add support for anonsubgifts

[sand-head]

I've been doing some digging, and it appears the msg-params-gift-months in that announcement (and apparently, in the docs for a while) is a typo. It's supposed to be msg-param-gift-months, to line up with all of the other tags.