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>

cgit.h

@@ -73,6 +73,7 @@ struct cgit_repo {
 	int enable_log_linecount;
 	int max_stats;
 	time_t mtime;
+	struct cgit_filter *about_filter;
 	struct cgit_filter *commit_filter;
 	struct cgit_filter *source_filter;
 };
@@ -185,6 +186,7 @@ struct cgit_config {
 	int summary_branches;
 	int summary_log;
 	int summary_tags;
+	struct cgit_filter *about_filter;
 	struct cgit_filter *commit_filter;
 	struct cgit_filter *source_filter;
 };