sliver-client's config directory hard-coded to ~/.sliver-client/

[lip234]

Is your feature request related to a problem? Please describe.
The config directory for sliver-client is hard-coded to ~/.sliver-client/, unlike sliver-server, whose config directory can be overridden using SLIVER_ROOT_DIR environment variable.

Describe the solution you'd like
Allow the config directory to be overridden by an environment variable like SLIVER_CLIENT_ROOT_DIR.

Describe alternatives you've considered
Perhaps also allow the config directory to be set by a command-line argument. Going this approach, sliver-server would also need to be updated for consistency.

Additional context
If this is something that you plan to implement, I'd be delighted to open a PR.