Add html_include()

This is a function used to include external htmlfiles in cgit-
generated pages.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -144,6 +144,7 @@ extern void html_hidden(char *name, char *value);
 extern void html_link_open(char *url, char *title, char *class);
 extern void html_link_close(void);
 extern void html_filemode(unsigned short mode);
+extern int html_include(const char *filename);
 
 extern int cgit_read_config(const char *filename, configfn fn);
 extern int cgit_parse_query(char *txt, configfn fn);