v0.20.0-alpha.1 - Prototype Performance Data release

NOTE: This is not considered a stable release.

While the plugins are likely safe to use, initial prototype support for emitting performance data has been added for the check_vmware_datastore plugin.

References:

• GH-315
• GH-321
• atc0005/go-nagios#81
• atc0005/go-nagios@1db1acf
• the (temporary) i321-prototype-datastore-perfdata-support branch
• commit 356565f