Microdata: allow users to toggle whether field is item or not

This is something that I've enabled as part of microdata module.

This includes the following changes:

• Update fivestar_microdata_value_type_alter to fivestar_microdata_value_types_alter.
• Change type from item to item_option
• Update suggested mapping so the field is handled as an item by default

I'm also moving the hook_microdata_suggestions implementation to the main module file. It can either be placed in the main module file or in a fivestar.microdata.inc file in the root directory, but it doesn't do anything where it is placed now.