Update package.json to add bin entry for walnut.

package.json

@@ -10,6 +10,9 @@
     "type": "git",
     "url": "https://github.com/Daplie/walnut.git"
   },
+  "bin": {
+    "walnut": "./bin/walnut.js"
+  },
   "keywords": [
     "ssl",
     "tls",