doc: change style for listings in asciidoc CSS
doc: use listing style for commands in quickstart guide
irc: fix prefix color for nick when the prefix is not in irc.color.nick_prefixes: use default color (key "*")

Problem was happening on a server which has "PREFIX=(Yqaohv)!~&@%+".
Users with prefix "!" were displayed as lightred (color for "~") instead of
lightblue (default key "*").

When a prefix was not found, WeeChat was looping on other prefixes (in
order). Now if color is not found, WeeChat uses immediately the fallback color.
irc: fix default completion (like nicks) in commands /msg, /notice, /query and /topic
ruby: fix crash with Ruby 2.0: use one array for the last 6 arguments of function config_new_option (bug #31050)