Comparison with document flow detection

Just saw that you (@garris) created a repo some time ago with _document flow detection_, resulting in a more reliable diffing than pixelmatch as any "vertical changes" are not automatically detected as visual differences.

Any plans to continue this and potentially integrate in Backstop? :)

https://github.com/garris/diverged

Start by reviewing the referenced diverged repository and how BackstopJS currently uses pixelmatch for visual comparison. Determine whether document flow detection can be integrated and define tests that demonstrate vertical changes are handled as intended; done means the approach is integrated with BackstopJS and its behavior is covered.

Written by the indexing model from the issue text.