Add 'loaders' option, as less-magical altenative to 'require' option. C…

…loses GH-8

The 'require' option allowed passing the require() function, which would
be pre-populated with require('pluginame') in the calling module so
browserify could statically analyze the module dependencies. This was
somewhat confusing since calling require() when ignoring the return
value has non-obvious side-effects (browserify static analysis):

    require('voxel-skyhook');

    pluginOpts = {
        'voxel-skyhook': {},
        ...
    }

The new 'loaders' option to voxel-plugins is more explicit:

    loaders = {
        'voxel-skyhook': require('voxel-skyhook'),
        ...
    },

where the specific require()'d modules are passed in as object values.
This is somewhat more typing, but is clearer and easier to follow. The
old require option is still supported for compatibility, but 'loaders' is
preferred for clarity.

plugins.js

@@ -14,6 +14,7 @@ function Plugins(game, opts) {
   if (this.game) this.game.plugins = this;
 
   opts = opts || {};
+  this.loaders = opts.loaders || {};
   this.require = opts.require || require;
   this.catchExceptions = false;
   this.masterPluginName = opts.masterPluginName || 'voxel-engine'; // synthetic 'plugin' created as parent of all
@@ -48,7 +49,13 @@ Plugins.prototype.wrapExceptions = function(f) {
 // Require the plugin module and return its factory constructor
 // This does not construct the plugin instance, for that see instantiate()
 Plugins.prototype.scan = function(name) {
-  var createPlugin = this.require(name);   // factory for constructor
+  var createPlugin; // factory for constructor
+
+  if (name in this.loaders) {
+    createPlugin = this.loaders[name];
+  } else {
+    createPlugin = this.require(name);
+  }
 
   return createPlugin;
 };