create clean task

Author: olegman

package.json

@@ -18,8 +18,9 @@
   "typings": "lib/index.d.ts",
   "scripts": {
     "postinstall": "typings install",
-    "watch": "rm -rf lib && tsc -p . --watch",
-    "transpile": "rm -rf lib && tsc -p ."
+    "clean": "rm -rf lib",
+    "watch": "npm run clean && tsc -p . --watch",
+    "transpile": "npm run clean && tsc -p ."
   },
   "devDependencies": {
     "typescript": "^2.0.3",