Filter for the values that appear in auto-suggest search

A [request from the forums](http://wordpress.org/support/topic/plugin-co-authors-plus-i-dont-want-my-users-can-see-other-members-email), the site admin should be able to filter which values appear to the end user in auto-suggest search.

The code around the auto-suggest JS is kinda ugly right now — we should probably fix that too.

No files or tests are named; start by tracing the auto-suggest JavaScript and the WordPress admin path that supplies its values. Define how the site admin controls which values are exposed, then verify that end users see only permitted values in search. The requested JavaScript cleanup needs separate scoping.

Written by the indexing model from the issue text.