adding support for "browser" field

Adding the "browser" field to package.json.
For details, see: https://github.com/defunctzombie/package-browser-field-spec

Useful when one is not using bower to install this package. Atleast webpack seems to pick this up without issue.
Webpack seems to support it - seems like an elegant solution for mixed usage libraries. 

Seems to be a better solution than #27 #27

package.json

@@ -33,6 +33,9 @@
     "release": "npm run upgrade && npm test && npm run bump && npm publish",
     "start": "npm run watch & http-server -o -c-1"
   },
+  "browser": {
+    "fs": false
+  },
   "repository": {
     "type": "git",
     "url": "https://github.com/BigstickCarpet/json-schema-ref-parser.git"
@@ -69,4 +72,4 @@
     "ono": "^2.2.1",
     "z-schema": "^3.17.0"
   }
-}
+}