Implement plain view

This implements a way to access plain blobs by path (similar to the
tree view) instead of by sha1.

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

cgit.h

@@ -165,6 +165,7 @@ struct cgit_config {
 struct cgit_page {
 	time_t modified;
 	time_t expires;
+	size_t size;
 	char *mimetype;
 	char *charset;
 	char *filename;