Add support for Analytics (Stats) API

[slavisah]

It would be great if the Analytics API could be supported.

Now it's possible to export some data to CSV, even though on GUI they offered much more. For example, URL for exporting some statistics about users could be constructed like this:

https://slack.com/stats/export?type=users&date_range=30d&username%2Cdays_active%2Cchats_sent&sort_prefix=chats_sent&sort_dir=desc&token={your_token}