Add option to only display users with specific IDs

[swissspidy]

The users should be also ordered according to these IDs. It doesn't look like WP_User_Query supports this, so we'd have to do the sorting ourselves.

[swissspidy]

Fixed in f7063dc

WP_User_Query already handles the comma separated lists. Ordering by IDs is possible by using orderby='include' in the shortcode.