exec: contribute "light" theme overrides

Add exec-theme.{c,h} with the exec plugin contribution to the built-in
"light" theme: 2 entries (exec.color.flag_finished=red,
exec.color.flag_running=green) tuned for a light-background terminal.

Same 2D-array pattern as the other plugin contributions.

Default option values are NOT changed.

src/plugins/exec/exec.c

@@ -32,6 +32,7 @@
 #include "exec-command.h"
 #include "exec-completion.h"
 #include "exec-config.h"
+#include "exec-theme.h"
 
 
 WEECHAT_PLUGIN_NAME(EXEC_PLUGIN_NAME);
@@ -742,6 +743,8 @@ weechat_plugin_init (struct t_weechat_plugin *plugin, int argc, char *argv[])
     if (!exec_config_init ())
         return WEECHAT_RC_ERROR;
 
+    exec_theme_init ();
+
     exec_config_read ();
 
     /* hook some signals */