Remove server for now (single-player only) - Update voxel-engine, remove voxel-fuel

Mainly to ease testing and porting, may re-add later. Plugins are now
loaded in voxel-engine itself, instead of the voxel-fuel wrapper (which
used to create two voxel-engine instances, for client and server). More
work is needed to get only server-only code loading in the server instance.

deathcap/voxel-engine@e4c579b8cf030d73323859fda0004c78c056ab05 Load materials from voxel-registry, if available, after loading plugins
deathcap/voxel-engine@9f85be8f5aca3c1ee871b9f042cb80cb37c123f7 Support voxel-engine 'plugin' for top-level options
deathcap/voxel-engine@fcdb36f85ef99c62dd8984e2407aaaad77732ce1 Add voxel-plugins support

Commit message generated by https://github.com/deathcap/lmno-cl