Support assigning multiple users

Per https://github.com/blog/2178-multiple-assignees-on-issues-and-pull-requests, GitHub now supports multiple assignees (up to 10) for a given issue.

Currently, ghi supports assigning only one user, or at least supports assigning only one user at a time:

$ ghi open --help
    [...]
    -u, --[no-]assign [<user>]       assign to specified user

Suggested approach is to simply allow multiple <user> arguments to to the -u/--assign option, e.g.

$ ghi open --help
    [...]
    -u, --[no-]assign <user>...      assign to specified user(s)

Likewise, it would be important to support un-assigning each specified user when the --no-assign option is used.