Ensure build output is cleaned before building

package.json

@@ -63,13 +63,14 @@
     "url-parse": "^1.5.7"
   },
   "scripts": {
+    "clean": "rm -rf dist lib.es5 lib.esm",
     "build-test-bundle": "mkdir -p dist && browserify test/spec/browser-index.js -t [ babelify --plugins [ @babel/transform-modules-commonjs ] ] -d -o dist/browser-test-bundle.js",
     "build-bundle": "mkdir -p dist && browserify lib/browser/index.js -t [ babelify --plugins [ @babel/transform-modules-commonjs ] ] -s tus -d | exorcist ./dist/tus.js.map > dist/tus.js",
     "build-minify": "uglifyjs ./dist/tus.js -o ./dist/tus.min.js --compress --mangle --source-map \"content='./dist/tus.js.map',url='tus.min.js.map'\"",
     "build-transpile-esm": "babel -d lib.esm/ lib/",
     "build-transpile-cjs": "babel --no-babelrc --plugins @babel/transform-modules-commonjs -d lib.es5/ lib.esm/",
     "build-transpile": "npm-run-all build-transpile-esm build-transpile-cjs",
-    "build": "npm-run-all build-bundle build-minify build-transpile build-test-bundle",
+    "build": "npm-run-all clean build-bundle build-minify build-transpile build-test-bundle",
     "watch-bundle": "chokidar --initial \"lib/**/*\" -c \"npm run build-bundle\"",
     "watch-test-bundle": "chokidar --initial \"test/spec/**/*\" -c \"npm run build-test-bundle\"",
     "watch-transpile": "chokidar --initial \"lib/**/*\" -c \"npm run build-transpile\"",