`form.select`, `options_for_select` and `options_from_collection_for_select`
now accept `priority`, `priority_separator` and `priority_unique` options.
When specified the given `priority` values will be placed at the top of the
select options, followed by all the options separated by the
`priority_separator` (which defaults to "-------------"). If `priority_unique`
is set to +true+ the values from the priority list will not be shown below
the separator. -- Issue #9620