Automatically detect sync conflicts

[OpenBagTwo]

GIVEN Steve has been careless about syncing his Ender Chests
AND has modified files within his Ender Chest on multiple systems sync his last sync

WHEN he next runs enderchest open or enderchest close between these systems

THEN EnderChest will detect when files on the destination system have been modified since the last sync[1]
AND will produce a report[2] on these conflicts
AND will ask Steve whether he would like to:

1. Abort
2. Proceed
3. Selectively proceed with only folders without conflicts
4. Selectively proceed with only specified folders[3]

SO that Steve doesn't have to manually merge the data

Notes

1. Note that this is not simply "has newer files"—the criterion is newer than last sync time (Get timestamp of last sync #137)[3]
2. As implied by the selectively-proceed options, this report should be per-folder (similar to the dry run report and what will be produced by Get timestep of last file changes #138)
3. Should there be an option to keep newest, though?
4. There should be both command-line flags and config options to disable this check or to set the default handling behavior
5. This issue is blocked by Get timestamp of last sync #137 and Get timestep of last file changes #138