Added possibility to use start= in force_navigate correctly

When I was trying to short-cut the navigation in this style:
```
if on detail page:
    start = "detail_page_location"
else:
    start = None
force_navigate("somewhere", start=start)
```
when the browser was 'shot in the head' due to bad alert or some exception, it tried to start from the starting location, therefore getting stuck in the fail loop because the start location was no longer valid. 

This should fix it - after browser is quitted, `start` is deleted from the parameters passed.