Support re-using data

[GuiltyDolphin]

Once a diff is produced, it would be handy if it could be re-used to produce a new diff with a new host (or newer version of an existing host) to allow comparisons over time.

For example, a bunch of screen shots may be produced on Beta, then a new version is deployed and we want to diff the new version with the old screen shots. We could specify the data file and the beta host and get an appropriate diff.

cc @jbarrett