extra_tablenav output for users should have a hook in it

[jakemgold]

In the WP_Users_List_Table class, the extra_tablenav method should really have an action hook in it to allow developers to add additional filters / navigation choices here.

Think of it as the equivalent to the "restrict_manage_posts" action hook for posts.

Hook added via patch

"extra_tablenav" is pretty internal. "restrict_manage_users" is consistent and simple.