Update to voxel-engine 0.20.0

Just a version update from voxel-engine 0.19.0 to 0.20.0. No code changes required since this version is backwards-compatible I believe (had to change this version since my peer dependencies wouldn't be satisfied..)

The other dependencies could be updated too (voxel-crunch from 0.1.1 to 0.2.1, duplex-emitter 0.1.10 to 2.1.2, extend 1.1.3 to 1.2.1) but those are larger version jumps, haven't tested their compatibility, but at least voxel-engine 0.19.0 to 0.20.0 should be compatible.