Add option to use only app/styles and includePath

Putting this here for discussion as it can help with adopted-ember-addons#74.

This PR adds a new option `onlyIncluded` which allows the user to prune the style tree down to just app/styles and anything in the `includePaths` array. This helps performance because the broccoli-caching-writer no longer has to stat everything in the addon trees (via walk-sync).  The speedup is especially pronounced when using linked modules because those trees can get very large.

index.js

@@ -1,6 +1,7 @@
 var SassCompiler = require('broccoli-sass-source-maps');
 var path = require('path');
 var checker = require('ember-cli-version-checker');
+var Funnel = require('broccli-funnel')
 var mergeTrees = require('broccoli-merge-trees');
 var merge = require('merge');
 var fs = require('fs');
@@ -13,8 +14,14 @@ function SASSPlugin(optionsFn) {
 
 SASSPlugin.prototype.toTree = function(tree, inputPath, outputPath, inputOptions) {
   var options = merge({}, this.optionsFn(), inputOptions);
-
-  var inputTrees = [tree];
+  var inputTrees
+
+  if (inputOptions.onlyIncluded) {
+    inputTrees = [new Funnel('app/styles', {destDir: 'app/styles'})];
+  } else {
+    inputTrees = [tree];
+  }
+
   if (options.includePaths) {
     inputTrees = inputTrees.concat(options.includePaths);
   }