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>

Makefile

@@ -55,6 +55,7 @@ OBJECTS += configfile.o
 OBJECTS += html.o
 OBJECTS += parsing.o
 OBJECTS += shared.o
+OBJECTS += ui-atom.o
 OBJECTS += ui-blob.o
 OBJECTS += ui-commit.o
 OBJECTS += ui-diff.o