irc: contribute "light" theme overrides

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.

src/plugins/irc/irc.c

@@ -47,6 +47,7 @@
 #include "irc-redirect.h"
 #include "irc-server.h"
 #include "irc-tag.h"
+#include "irc-theme.h"
 #include "irc-typing.h"
 #include "irc-upgrade.h"
 
@@ -217,6 +218,8 @@ weechat_plugin_init (struct t_weechat_plugin *plugin, int argc, char *argv[])
 
     irc_config_read ();
 
+    irc_theme_init ();
+
     irc_list_init ();
 
     irc_raw_init ();