Append before checking option

[Evengard]

This is another take on the problem possibly solvable by #15 .

It would be nice that if diskrsync had an option (eg --append-before-checking), which would implement this behaviour:

If diskrsync detects that the source file is bigger than the target file - first of all append all "missing" data to the target file until their sizes match, and only then do the checksum sweep and downloading all the mismatching blocks. This would make it a lot easier for resuming broken uploads without the need to check for all data when we know that the file size doesn't match yet.