Support creating tags from custom properties

[Moguri]

Custom properties are another way (in addition to game properties) to add arbitrary information to objects. With the game engine being removed in 2.80, custom properties will become the only way to get custom data to export as tags. Custom properties also exist in 2.7x, so support can be added in anticipation of 2.8x support.

[ArsThaumaturgis]

The link above seems to be broken; does the following link describe what you're referring to?

https://docs.blender.org/manual/en/dev/files/data_blocks.html#custom-properties