It would be nice to be able to specify custom providers using new providers files instead of editing the default one.

e.g.

Instead of adding:

https://my.op my-client-id 24h

to /etc/opk/providers

We could instead create a new file

/etc/opk/providers.d/my-op

that contains

https://my.op my-client-id 24h

opkssh verify could then first read /etc/opk/providers then any files in /etc/opk/providers.d

Unsure if this clashes with #60 or not