support .vpd format

It would be nice to also support Vocaloid Pose Data (abbreviated as VPD), which is a single framed text-based format, just like vmd.

Examples:
https://learnmmd.com/ReggiesPoseCollection.zip?amp=1

https://github.com/mfakane/MikuMikuMoving-Plugins/blob/1a9cdf6990fa5dcd1e4bac3f714f4663fb220e85/SetMmdTransformationPlugin/Vpd/VpdDocument.cs#L12