Clean lib directory before build

Author: samuelmeuli

package.json

@@ -22,6 +22,7 @@
 		"start:rollup": "rollup --config --sourcemap --watch",
 		"start:storybook": "start-storybook --port 3000",
 		"build": "run-s build:*",
+		"build:clean": "rm -rf lib",
 		"build:rollup": "rollup --config",
 		"build:storybook": "build-storybook --output-dir dist",
 		"lint": "eslint ."