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.h

@@ -36,6 +36,7 @@ struct repoinfo {
 	char *desc;
 	char *owner;
 	char *defbranch;
+	char *group;
 	char *module_link;
 	int snapshots;
 	int enable_log_filecount;
@@ -80,6 +81,7 @@ extern char *cgit_module_link;
 extern char *cgit_virtual_root;
 extern char *cgit_script_name;
 extern char *cgit_cache_root;
+extern char *cgit_repo_group;
 
 extern int cgit_nocache;
 extern int cgit_snapshots;