Add option to exclude blendshapes or use sparse accessors for 5x file size reduction

[hybridherbst]

Currently a set of blendshapes is always included. This increases filesize 5x (from ~2 MB to > 10 MB) and isn't always necessary - many apps can't do anything with them.

The alternative to removing them is to always use sparse accessors, which reduces to ~2.5 MB which would be acceptable. Right now sparse accessors are not used in the downloaded files it seems.