Switch to pug-inheritance and changed default file extension to .pug

Author: pure180

package.json

@@ -36,7 +36,7 @@
   "dependencies": {
     "event-stream": "^3.3.4",
     "gulp-util": "^3.0.7",
-    "pug-inheritance": "^1.2.0",
+    "pug-inheritance": "^2.0.0",
     "lodash": "^4.16.4",
     "through2": "^2.0.1",
     "vinyl-fs": "^2.4.3"