@see: slackapi/hubot-slack#105 (comment)
I needed this functionality too, so I ended up working around this by writing a standalone PHP script
to automatically invite the bot into every channel. If I get time I'll convert it into node.js.
It essentially just uses the Web API to pull down a list of all channels:
https://slack.com/api/channels.list?token=MY_TOKEN
This gives me the Slack ID for every channel (ie "A12B45C789"), as well as which Users are members of that Channel.
I can then send a second API request to invite my bot into every channel which it isn't already a member of.
https://slack.com/api/channels.invite?token=MY_TOKEN&channel=A12B45C789&user=U123456789
(the user parameter is the Slack ID of my bot).
It's clunky but it works. Currently running it as a scheduled cron job but would eventually prefer to use
a webhook (listening for a channel created event).