Adding support for running external command before and after each backup.

Both commands have a configurable timeout value. Additionally, the pre-backup command can be tagged as critical, in which case bvckup checks the exit code and aborts the backup if the code doesn't match specified value.

The screenshot shows touching a TrueCrypt volume before the backup. This is needed, because TrueCrypt goes out of its way to preserve timestamps on its container files and bvckup sees such files as unmodified.