Re-worked plugin and fieldtype to play nicely together.

- Field now returns the proper shipping rate if the plugin is enabled
- Adheres to CartThrob's "number formatting" options
- {field:number} to get just the number
- To check for free shipping — {if "{field:number}" > 0} — must use quotes because an empty channel field will simply return a blank string, I can't force it to return 0.

Signed-off-by: Jeston Tigchon <jeston@cullgroup.com>