Update for magic-free plugin loading

[deathcap]

Currently voxel-example uses the implicit require() calls for browserify and passes the module's own require function:

var createEngine = require('voxel-engine-stackgl')

require('./lib/blocks.js')
require('./lib/terrain.js')

var main = function() {
  createEngine({require: require, exposeGlobal: true, pluginOpts: {
    'game-shell-fps-camera': {position: [-4, -40, -5], rotationX:15*Math.PI/180, rotationY:135*Math.PI/180 },

    './lib/blocks.js': {},
    './lib/terrain.js': {}
  }})
};

module.exports = main

This still works but voxel-plugins 0.5.0+ gained a more straightforward loading mechanism voxel/voxel-plugins#8 where the plugin constructors are passed in a loaders object instead of through a require function; voxel-example should update to use it.

[deathcap]

Updated to use pluginLoaders in 9d27550