Clean output directory on build

Install rimraf for cross-OS compatible directory deletion.

Author: wimpyprogrammer

package.json

@@ -9,13 +9,15 @@
     "lib": "lib"
   },
   "scripts": {
-    "build": "babel src -d lib",
+    "build": "npm run clean && babel src -d lib",
+    "clean": "rimraf lib/*",
     "test": "echo \"Error: no test specified\" && exit 1"
   },
   "dependencies": {},
   "devDependencies": {
     "@babel/cli": "^7.0.0-beta.54",
     "@babel/core": "^7.0.0-beta.54",
-    "@babel/preset-env": "^7.0.0-beta.54"
+    "@babel/preset-env": "^7.0.0-beta.54",
+    "rimraf": "^2.6.2"
   }
 }

yarn.lock

@@ -1502,7 +1502,7 @@ ret@~0.1.10:
   version "0.1.15"
   resolved "https://registry.yarnpkg.com/ret/-/ret-0.1.15.tgz#b8a4825d5bdb1fc3f6f53c2bc33f81388681c7bc"
 
-rimraf@^2.6.1:
+rimraf@^2.6.1, rimraf@^2.6.2:
   version "2.6.2"
   resolved "https://registry.yarnpkg.com/rimraf/-/rimraf-2.6.2.tgz#2ed8150d24a16ea8651e6d6ef0f47c4158ce7a36"
   dependencies: