Teach cgit how to group repositories by category

The new parameter 'repo.group' is used to set the repository group
for the following repositores. Whenever this parameter changes value,
a subheading is generated in the index page (printing the current value
of repo.group).

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

cgit.css

@@ -353,3 +353,8 @@ table.diff td div.del {
 .right {
 	text-align: right;
 }
+
+table.list td.repogroup {
+	padding-top: 1em;
+	font-weight: bold;
+}