chore(build): add clean script

removes build artifacts

Author: adriancarriger

package.json

@@ -9,6 +9,7 @@
     "commit": "npm run prepublish && npm test && git-cz",
     "prepublish": "ngc && npm run build",
     "build": "webpack && cp bundles/angularfire2-offline.umd.js bundles/index.js",
+    "clean": "rimraf index.js ./*{.d.ts,offline.js,.metadata.json} ./src/**/*.{d.ts,js,metadata.json}",
     "semantic-release": "semantic-release pre && npm publish --tag=next && semantic-release post"
   },
   "repository": {
@@ -71,6 +72,7 @@
     "loader-utils": "0.2.16",
     "localforage": "1.5.0",
     "reflect-metadata": "0.1.8",
+    "rimraf": "^2.6.1",
     "rxjs": "5.0.3",
     "semantic-release": "^6.3.2",
     "source-map-loader": "0.1.5",

yarn.lock

@@ -3439,6 +3439,12 @@ rimraf@2, rimraf@^2.3.3, rimraf@^2.4.4, rimraf@~2.5.1, rimraf@~2.5.4:
   dependencies:
     glob "^7.0.5"
 
+rimraf@^2.6.1:
+  version "2.6.1"
+  resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-2.6.1.tgz#c2338ec643df7a1b7fe5c54fa86f57428a55f33d"
+  dependencies:
+    glob "^7.0.5"
+
 ripemd160@^1.0.0:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/ripemd160/-/ripemd160-1.0.1.tgz#93a4bbd4942bc574b69a8fa57c71de10ecca7d6e"