Add support for pattern prefix for the whole file with --exclude-from

Given that you have a list of regular expression patterns already (e.g. used with "bup"), it would be helpful if `--exclude-from re:/etc/backup-exclude-bup.txt` would work, i.e. consider all patterns in the file to have a `re:` prefix.

I've used the following Vim commands to convert the list manually:

``` vim
:%s/^\ze[^#]/re:/gc
" borg does not like a / at the end before $ to exclude dirs?!
:%s~/\$~\$~gc
```