How to run the compare step manually?

Hey there,

I've been using BackstopJS in my project with great success but I'm at the point where I'd like to compare screenshots manually. For example, I've been saving the screen shots from my project using the `backstop test` command and placed them in a repo organised by the project's `BRANCH_NAME/COMMIT_SHA`. Now what I would like to do is grab a set of screen shots from any particular commit, compare them against another set and generate that html report.

I've tried to fiddle around with the internals of BackstopJS but I'm a little over my head with trying to just get the compare step to work. Has anyone gotten something like that to work?