Add mg_http_serve_opts.page404

src/http.h

@@ -26,6 +26,7 @@ struct mg_http_serve_opts {
   const char *ssi_pattern;    // SSI file name pattern, e.g. #.shtml
   const char *extra_headers;  // Extra HTTP headers to add in responses
   const char *mime_types;     // Extra mime types, ext1=type1,ext2=type2,..
+  const char *page404;        // Path to the 404 page, or NULL by default
   struct mg_fs *fs;           // Filesystem implementation. Use NULL for POSIX
 };