add forge as dep, ursa as optional

package.json

@@ -34,5 +34,11 @@
   "bugs": {
     "url": "https://github.com/Daplie/rsa-compat.js/issues"
   },
-  "homepage": "https://github.com/Daplie/rsa-compat.js#readme"
+  "homepage": "https://github.com/Daplie/rsa-compat.js#readme",
+  "dependencies": {
+    "node-forge": "^0.6.41"
+  },
+  "optionalDependencies": {
+    "ursa": "^0.9.4"
+  }
 }