Error: ENAMETOOLONG (url too long)

Hi there,

While taking screenshots of a website that has very long urls, the following error showed up:

```
Error: ENAMETOOLONG: name too long, open '/path/to/backstopjs/backstop_data/bitmaps_test/20180522-154944/failed_diff_backstop_default__provost_undergraduate-affairs_advising-network_undergraduate-academic-advising-awards_2016-undergraduate-academic-advising-awards-committee-intranet_smith-michael-2016-faculty-academic-advisor-award-candidate__0_document_0_laptop.png'
```

We have no control over the urls themselves so we can't just shorten them =/

Is there anyway to make backstopjs handle really long urls in different way? maybe truncate the file name? maybe use a hash for the file name? maybe both?

Thanks,
Carlos

Reproduce the screenshot case with the long URL shown in the error and trace where the failed-diff output filename is generated. The issue does not name an implementation file or test; done means long URLs can be processed without ENAMETOOLONG while keeping the resulting screenshot artifacts distinguishable.

Written by the indexing model from the issue text.