prefix(prefix) {
    prefix = prefix.replace(/\/$/, '');

    this.opts.prefix = prefix;

    for (let i = 0; i < this.stack.length; i++) {
      const route = this.stack[i];
      route.setPrefix(prefix);
    }

    return this;
  }
Set the path prefix for a Router instance that was already initialized.

@example

```javascript
router.prefix('/things/:thing_id')
```

@param {String} prefix
@returns {Router}