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>

ui-shared.h

@@ -1,6 +1,7 @@
 #ifndef UI_SHARED_H
 #define UI_SHARED_H
 
+extern char *cgit_hosturl();
 extern char *cgit_repourl(const char *reponame);
 extern char *cgit_fileurl(const char *reponame, const char *pagename,
 			  const char *filename, const char *query);