fix(ui): don't overwrite default options

Object.assign writes values from args[1+] into args[0] so defaultOptions were being overridden

lib/ui/index.js

@@ -35,7 +35,7 @@ class UI {
      * @param {Object} options
      */
     constructor(options) {
-        this.options = Object.assign(defaultOptions, options);
+        this.options = Object.assign({}, defaultOptions, options);
 
         // Set up i/o streams
         this.stdin = this.options.stdin;