Add atom-support

This enables a page which generates atom feeds for the current branch and
path, heavily inspired by the atom-support in gitweb.

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

cgit.h

@@ -24,6 +24,7 @@
  */
 #define FMT_LONGDATE "%Y-%m-%d %H:%M:%S (%Z)"
 #define FMT_SHORTDATE "%Y-%m-%d"
+#define FMT_ATOMDATE "%Y-%m-%dT%H:%M:%SZ"
 
 
 /*