Add irc-theme.{c,h} with the IRC plugin contribution to the built-in
"light" theme: 9 overrides on irc.color.* options tuned for a
light-background terminal (input_nick, item_lag_finished,
item_tls_version_deprecated, list_buffer_line_selected,
message_chghost, message_setname, nick_prefixes, topic_new, topic_old).
The registration is a small wrapper around weechat_theme_register that
builds a hashtable from a static (option, value) table and frees it
after the call. irc_theme_init() is called from weechat_plugin_init
after irc_config_init/read so the option names are already created
when the theme registry references them.
Default option values are NOT changed.
Sébastien Helleu