Add support for "readme" option

The value of this option is used as the default value for repo.readme.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/scan-tree.c b/scan-tree.c
index e987824..780d405 100644
--- a/scan-tree.c
+++ b/scan-tree.c
@@ -110,9 +110,11 @@
 	if (!stat(p, &st))
 		readfile(p, &repo->desc, &size);
 
-	p = fmt("%s/README.html", path);
-	if (!stat(p, &st))
-		repo->readme = "README.html";
+	if (!repo->readme) {
+		p = fmt("%s/README.html", path);
+		if (!stat(p, &st))
+			repo->readme = "README.html";
+	}
 
 	p = fmt("%s/cgitrc", path);
 	if (!stat(p, &st)) {