fix(package): update yargs to version 15.3.0

Closes #80

Author: greenkeeper[bot]

package.json

@@ -77,6 +77,6 @@
     "reflect-metadata": "^0.1.13",
     "showdown": "^1.9.0",
     "xml2js": "^0.4.19",
-    "yargs": "^14.0.0"
+    "yargs": "^15.3.0"
   }
 }