Make UpdateUserGroupMembers arguments to be more consistent

[painhardcore]

Change is pretty simple and needed to make flow more straightforward
Get members from group --> Update group with members
Get slice --> send slice
closes #1172

[johanmcos]

I'm not a maintainer, but I think if you want any chance of this being accepted, you're going to need to figure out how to make your change backwards-compatible.

Perhaps you could use a variadic function argument for members

[karnikg]

This perhaps is not a needed/relevant change? I looked at forking the repo to make this change, and realized from the documentation that the actual API needs a comma separated string

A comma separated string of encoded user IDs that represent the entire list of users for the user group.

https://api.slack.com/methods/usergroups.users.update
Passing a comma separated string instead seemed to work just fine for me (instead of a list)