Feature request: add possibility to launch differs blocking

[obestwalter]

As seen in #93 there are tools that make it necessary to run the differ blocking to even be able to use approvaltests.

I would also add that it might be a workflow preference to have the differ block until it is close by the user to be able to work through a series of failures one by one rather than to have them pop open all at the same time. This is definitely a mater of taste though, but together with the above problem could make the case that a possibility to configure might make sense.

I would be willing to prepare (or help preparing) a patch.