fn process_getconf(conf: &mut WeldConf, key: &str) -> Option<String> {
    conf.get(key).cloned().map(|e| e.into_string().unwrap())
}
Process the `GetConf` command.

The argument is a key in the configuration. The command returns the value of the key or `None`
if no value is set.
weld-repl/src/main.rs