Allow subclasses to add additional input nodes.

Subclassing broccoli-funnel has become somewhat common (e.g. Embroider
and ember-css-modules both do it). This adds a basic test to confirm
that it generally works.

Additionally, this adds the ability for the subclass to add additional
nodes to be considered as part of the broccoli tree graph. This ensures
that if you happen to subclass and need to read from another tree to
know where to write, you can add that other tree to your inputs and be
*guaranteed* that they will have been built before you are called.
Without this change, it _might_ seem to work **sometimes** but it is
absolutely not guaranteed to work.

index.js

@@ -50,8 +50,9 @@ function existsSync(path) {
 }
 
 class Funnel extends Plugin {
-  constructor(inputNode, options = {}) {
-    super([inputNode], {
+  constructor(inputs, options = {}) {
+    let inputNodes = Array.isArray(inputs) ? inputs : [inputs];
+    super(inputNodes, {
       annotation: options.annotation,
       persistentOutput: true,
       needsCache: false,

tests/index.js

@@ -1106,4 +1106,66 @@ describe('broccoli-funnel', function() {
       expect(getDestPathCalled).to.be.equal(1);
     });
   });
+
+  describe('subclassing', function() {
+    it('can be subclassed, simple destDir modification', async function() {
+      class FunnelSubclass extends Funnel.Funnel {
+        constructor(input, options) {
+          super(input, options);
+
+          this._hasBuilt = false;
+        }
+
+        build() {
+          if (this._hasBuilt === false) {
+            this.destDir = 'lol';
+            this._hasBuilt = true;
+          }
+
+          return super.build();
+        }
+      }
+
+      let inputPath = input.path('lib/utils');
+      let node = new FunnelSubclass(inputPath, {});
+      output = createBuilder(node);
+
+      await output.build();
+      let outputPath = output.path();
+
+      expect(walkSync(outputPath)).to.eql(['lol/', 'lol/foo.js']);
+    });
+
+    it('subclasses can provide additional trees', async function() {
+      class FunnelSubclass extends Funnel.Funnel {
+        constructor(inputNode, options) {
+          super([inputNode, input.path('dir1/subdir2')], options);
+
+          this._hasBuilt = false;
+        }
+
+        build() {
+          if (this._hasBuilt === false) {
+            if (!fs.existsSync(`${this.inputPaths[1]}/bar.css`)) {
+              throw new Error('Could not find file!!!');
+            }
+            // set custom destDir to ensure our custom build code ran
+            this.destDir = 'lol';
+            this._hasBuilt = true;
+          }
+
+          return super.build();
+        }
+      }
+
+      let inputPath = input.path('lib/utils');
+      let node = new FunnelSubclass(inputPath, {});
+      output = createBuilder(node);
+
+      await output.build();
+      let outputPath = output.path();
+
+      expect(walkSync(outputPath)).to.eql(['lol/', 'lol/foo.js']);
+    });
+  });
 });