Permit overriding global forwarders with an empty list (per zone)

[jjthiessen]

This PR:

• Expands on the zone types that are permitted to have forward and forwarders directives (ref)
• Allows for overriding global forwarders with an empty list on a per zone basis
• Sneaks in some reformatting/re-indenting of templates/zone.erb for consistency and cleaner looking generated zone files

[jjthiessen]

This addresses #204 (like #210), but would require a major version bump due to backwards incompatible behaviours.