Feature Request: Add --sslca= to command line options.

[Splediferous]

During the discussion for SSL in 2014, this (the command-line option) was requested as an option. While the other options have been implemented, I would like to request the additional command-line option "--sslca=".

I tried the option in version 0.4.1 but it was not implemented; unless I am missing something.

Use case:
A public webserver, with a signed certificate from an an approved CA, limits websocket access to known, self-signed, client certificates. Developers create their own key-pairs and supply the public certificate to IT for websocket access.

Problem:
Currently, the software requires client certificates signed by an approved certificate authority since the CA list used is system wide. Therefore the websocket server refuses connections from self-signed certificates. with the following error:
TLS handshake error from ********:****: remote error: tls: unknown certificate authority
To gain the function, modification of the system-wide certificate store to add the clients' certificate is required. The system-wide certificate store is automatically updated regularly (e.g. by Lets Encrypt or IT), overwriting the changes, and modification of the vanilla CA file is inherently a security risk, changing it's verifiable hash.

Resolution:
Use a specific CA file only listing allowed clients' public certificates, obviating the need to modify system-wide CA files.
This would allow the clients to verify the public server against the sites public certificate (by an approved CA) while accepting known clients identified by their self-signed certificate. I believe this would require a command-line or file configuration setting to define the certificate store to use. (e.g. --sslca=).

Nice-to-haves:
The ability to define a directory of certificates rather than a single file for --sslca=.