Gulp: added 'cleanup' task that erases all gulp-generated files.

It uses Git to erase those files, so anything that's tracked in Git (such
as the JS/CSS vendor directories) are kept as-is.

package.json

@@ -13,6 +13,7 @@
     "gulp-chmod": "~1.3.0",
     "gulp-concat": "~2.6.0",
     "gulp-if": "^2.0.1",
+    "gulp-git": "~2.4.2",
     "gulp-livereload": "~3.8.1",
     "gulp-plumber": "~1.1.0",
     "gulp-pug": "~3.2.0",