Add watch script for dev (#234)

.babelrc

@@ -10,6 +10,14 @@
     "transform-flow-strip-types"
   ],
   "env": {
+    "watch": {
+      "presets": [
+        ["es2015", {
+          "loose": true
+        }],
+        "stage-0"
+      ]
+    },
     "test": {
       "plugins": ["istanbul"]
     }

package.json

@@ -40,17 +40,18 @@
     "babylon": "./bin/babylon.js"
   },
   "scripts": {
-    "build": "rollup -c",
+    "build": "npm run clean && rollup -c",
     "coverage": "nyc report --reporter=json && codecov -f coverage/coverage-final.json",
     "lint": "eslint src bin",
     "clean": "rimraf lib",
     "flow": "flow",
-    "prepublish": "npm run clean && cross-env BABEL_ENV=production npm run build",
+    "prepublish": "cross-env BABEL_ENV=production npm run build",
     "preversion": "npm run test && npm run changelog",
     "test": "npm run lint && npm run flow && npm run build -- -m && npm run test-only",
     "test-only": "ava test",
     "test-ci": "nyc npm run test-only",
-    "changelog": "git log `git describe --tags --abbrev=0`..HEAD --pretty=format:' * %s (%an)' | grep -v 'Merge pull request'"
+    "changelog": "git log `git describe --tags --abbrev=0`..HEAD --pretty=format:' * %s (%an)' | grep -v 'Merge pull request'",
+    "watch": "npm run clean && cross-env BABEL_ENV=watch babel src --out-dir lib --watch"
   },
   "nyc": {
     "include": [