Enable toggling on fields to handle value as item

Motivation

Some fields, such as taxonomy references, should be handled as string-valued properties some times and as item-valued properties other times. Sometimes the module developer will be the one doing this (using hook_microdata_value_type_alter), but many times it will be the user who is configuring the site.

Proposed Solution

Offer a checkbox for any field to optionally handle it as an item.