fix(encrypt cli): use comma separated list when encrypting files for multiple recipients

I just came across something that might not be intuitive, but I think is correct syntax-wise:

sett encrypt -r jaroslaw.surkont@unibas.ch docs/

doesn't work, docs is interpreted as the second recipient:

sett encrypt -r jaroslaw.surkont@unibas.ch -- docs/

works fine.

Fix: use comma separated list of recipients?

sett encrypt -r jaroslaw.surkont@unibas.ch,robin.engler@sib.swiss docs/