Add support for multiple users on Channels Invite endpoint

[abmartinr]

Hey guys!

Amazing job on this Gem! I love it :)

One thing I'd like to request if possible is to add support for multiple users on the channels invite endpoint. Slack supports up to 30 users on this endpoint if you pass users instead of user.

Thanks!

[dblock]

It doesn't look like it's a documented feature, I'm looking at https://api.slack.com/methods/channels.invite. The library is auto-generated from that via https://github.com/slack-ruby/slack-api-ref. I opened slack-ruby/slack-api-ref#6, please feel free to contribute.