Make users.list accept pagination args

[xiaochuanyu]

This PR adds cursor and limit to users.list API to allow pagination per https://api.slack.com/docs/pagination. API should be backward compatible. I also modified example to show how paging is done. I removed the channels part in example because that API is deprecated.

[CarltonHowell]

@os Any chance we can get some 👀 on this? 🙏