Stop startup on a list or map setting in neoirc.yaml (closes #268)

Every setting is now read through one helper that uses cast.ToStringE,
the conversion viper.GetString wraps, keeping its error instead of
returning the empty string. TRUSTED_PROXIES uses the same helper. A
config file that does not parse makes New return an error naming the
file instead of panicking.

Model: opus-5-5

README.md

@@ -1722,7 +1722,8 @@ exceptions:
 
 A `neoirc.yaml` that is not valid YAML stops the server at startup. A setting
 that does not parse or is out of range also stops the server at startup, and the
-error names the setting.
+error names the setting. The same holds for a setting given a list or a map in
+`neoirc.yaml`.
 
 - `PORT` (default `8080`): the HTTP port. `0` lets the system pick a free port.
 - `IRC_LISTEN_ADDR` (default `:6667`): the address of the IRC listener. The