#45 Support resuming merge operation

[Andy Gianfagna]

Sometimes I find myself comparing and merging several sets of files, but then cannot complete the operation before I need to shutdown my computer. It would be really handy if there was a way to save the current work in progress and allow you to re-open all of the files that you were currently working with again at a later point in time. Maybe this could just write the names of each file to some output file, which when read back in would just reload those files in the same order.