Fix StitchError stack traces in node

* Add `babel-plugin-transform-builtin-extend`
* Subclassing builtin types needs special treatment by babel
  * Without this plugin, in a node environment, a subclass of Error does not actually behave like a true subclass, which manifests itself via a missing stacktrace when a StitchError is thrown

More details [here](babel/babel#3083)

.babelrc

@@ -6,11 +6,12 @@
         "node": "6"
       },
       "useBuiltins": "usage",
-    }]
+    }],
   ],
   "plugins": [
     "transform-async-to-generator",
     "babel-plugin-add-module-exports",
-    "version-transform"
+    "version-transform",
+    ["babel-plugin-transform-builtin-extend", { globals: ["Error"] }]
   ]
 }

package.json

@@ -40,6 +40,7 @@
     "babel-loader": "^7.0.0",
     "babel-plugin-add-module-exports": "^0.2.1",
     "babel-plugin-transform-async-to-generator": "^6.24.1",
+    "babel-plugin-transform-builtin-extend": "^1.1.2",
     "babel-preset-env": "^1.6.1",
     "conventional-changelog-cli": "^1.3.1",
     "eslint": "^3.19.0",

test/errors.test.js

@@ -0,0 +1,9 @@
+import { StitchError } from '../src/errors';
+
+describe('StitchError', () => {
+  it('reports a full stack trace', () => {
+    const err = new StitchError("Oh noes!");
+    expect(err.toString()).toEqual('StitchError: Oh noes!');
+    expect(err.stack.length).toBeGreaterThan(err.toString().length);
+  });
+});