Multiple `proxy_ca_certs`?

I was wondering if its possible to add multiple `proxy_ca_certs`. Also the documentation for pretty much all configuration in `config.json` is really poorly documented: the only resources I could find on custom CAs was a GitHub issue.

I tried setting `proxy_ca_certs` to an array, but it only seems to be doing one of them.

When cert no. 1 is not loaded, I get the error
```
▶ ERROR API network error: Post https://api-0.core.keybaseapi.com/_/api/1.0/ping.json: x509: certificate signed by unknown authority
```

And when cert no. 2 isn't, I get this
```
▶ WARNING (CONN gregor 5da09c1a) Connection: error dialing transport: x509: certificate signed by unknown authority
```

Using neither cert results in both errors. Some trial and error has caused me to think that only the last cert is being used? I'm not completely sure.

Start with the config.json handling for proxy_ca_certs and trace how the configured certificates are used for the API and Gregor connections. Done means multiple configured CA certificates are all honored, and the config.json documentation explains the supported format and behavior.

Written by the indexing model from the issue text.