Provide an alternative way to specify DSN

```
● klaes.service - PGP Key Server
Loaded: loaded (/etc/systemd/system/klaes.service; disabled; vendor preset: enabled)
Active: active (running) since Wed 2020-03-25 23:46:11 UTC; 9s ago
Main PID: 29580 (klaes)
Tasks: 7 (limit: 2320)
CGroup: /system.slice/klaes.service
└─29580 /usr/local/bin/klaes -addr :11371 -sql-driver postgres -sql-source host=[REDACTED].hex.dn42 user=klaes dbname=klaes sslmode=disable password=[REDACTED] serve

```
That's .. not optimal.