ui-blame: add blame UI

Implement a page which provides the blame view of a specified file.

This feature is controlled by a new config variable, "enable-blame",
which is disabled by default.

Signed-off-by: Jeff Smith <whydoubt@gmail.com>
Reviewed-by: John Keeping <john@keeping.me.uk>

cgit.mk

@@ -77,6 +77,7 @@ CGIT_OBJ_NAMES += parsing.o
 CGIT_OBJ_NAMES += scan-tree.o
 CGIT_OBJ_NAMES += shared.o
 CGIT_OBJ_NAMES += ui-atom.o
+CGIT_OBJ_NAMES += ui-blame.o
 CGIT_OBJ_NAMES += ui-blob.o
 CGIT_OBJ_NAMES += ui-clone.o
 CGIT_OBJ_NAMES += ui-commit.o