Add 'about-filter' and 'repo.about-filter' options

These options can be used to execute a filter command on each about-page,
both top-level and for each repository (repo.about-filter can be used
to override the current about-filter).

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

ui-repolist.c

@@ -273,6 +273,11 @@ void cgit_print_repolist()
 
 void cgit_print_site_readme()
 {
-	if (ctx.cfg.root_readme)
-		html_include(ctx.cfg.root_readme);
+	if (!ctx.cfg.root_readme)
+		return;
+	if (ctx.cfg.about_filter)
+		cgit_open_filter(ctx.cfg.about_filter);
+	html_include(ctx.cfg.root_readme);
+	if (ctx.cfg.about_filter)
+		cgit_close_filter(ctx.cfg.about_filter);
 }