Possibility to create PKCS12 file

[mharrend]

First of all thanks for the very useful package.

For one application (Thingsboard) I would need a pkcs12 certificate store instead of PEM files.
Currently, I am using a cron job in /etc/periodic/monthly to do this via

cd /etc/letsencrypt/production/certs/thingsboard/ 
openssl pkcs12 -export -in fullchain.pem -inkey privkey.pem -out pkcs.p12 -name thingsboard -password pass:PASSWORD

However, this requires to mount /etc/periodic as a volume on the host to keep the cron job persistent.

It would be nice to include this in the binary rancher-letsencrypt.
Maybe in this place:
https://github.com/janeczku/rancher-letsencrypt/blob/master/manager.go#L128
via os/exec.
The reason of using os/exec would be that unfortunately encoding pkcs12 file never made it into x/crypto repo, see golang/go#14125

Would you except a PR in such a manner? If yes, I would work on such one.