Using BackstopJS with URL Parameters

Hi,

Thanks for a great tool. I am setting up some regression tests for a PatternLab installation. The URLs I have to work with are like so:
`http://localsite.dev/themes/custom/patternlab/pattern-lab/public/?p=sample-pages-basic-page`

It seems I can easily test like this:

```
{
"label": "01 Homepage",
"url": "http://localsite.dev"
},
```

But I don't want to test that page, as the content might change in the CMS so my tests will fail. Instead I'd like to test my static version of the page, like this:

```
{
"label": "01 Homepage",
"url": "http://localsite.dev/themes/custom/patternlab/pattern-lab/public/?p=sample-pages-basic-page"
},
```

However, it seems to fail on me. If I set the path directly to the html file it works, so I have a workaround - it's just tedious to keep trying to find the file (much simpler to copy the URL from the browser).

```
{
"label": "01 Homepage",
"url": "../../../web/themes/custom/patternlab/pattern-lab/public/patterns/06-sample-pages-basic-pages-basic-page/06-sample-pages-basic-pages-basic-page.html",
},
```

Thanks for any help.