MINOR: spoa-server: Load files

Declare files to be executed at the begining and execute it. The binding
between the engine and the file is done throught the extension.
diff --git a/contrib/spoa_server/spoa.h b/contrib/spoa_server/spoa.h
index 22d6788..ca85181 100644
--- a/contrib/spoa_server/spoa.h
+++ b/contrib/spoa_server/spoa.h
@@ -90,6 +90,7 @@
 	char *ext;
 	int (*init_worker)(struct worker *w);
 	int (*exec_message)(struct worker *w, void *ref, int nargs, struct spoe_kv *args);
+	int (*load_file)(struct worker *w, const char *file);
 };
 
 struct ps_message {