implement more rsync flags for compatibility

[stapelberg]

In general, we should try to be compatible where reasonable (i.e. where it isn’t too much effort and the use-case seems clear).

Simon tells me that popular RPKI software (like rpki-client or routinator 3000) seem to expect the following options:

• --delete, see issue implement --delete flag #34
• --omit-dir-times
• --contimeout (routinator 3000 default if detected)
• --no-motd (routinator 3000 default)
• --compress, -z (routinator 3000 default)
• --min-size, --max-size
• --timeout
• --include
• --exclude