Add submodule links in tree listing

When a submodule occurs in a tree, generate a link to show the
module/commit. The link is specified as a sprintf string in /etc/cgitrc,
using parameters 'module-link' and 'repo.module-link'. This should probably
be extended with repo.module-link.$path.

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

cgit.css

@@ -141,7 +141,7 @@ div.error {
 	margin: 1em 2em;
 }
 
-div.ls-blob, div.ls-dir {
+td.ls-blob, td.ls-dir, td.ls-mod {
 	font-family: monospace;
 }