ui-tree: add support for source-filter option

This new option is used to specify an external command which will be
executed when displaying blob content in the tree view. Blob content
will be written to STDIN of the filter and STDOUT from the filter
will be included verbatim in the html output from cgit. The file name
of the blob will be passed as the only argument to the filter command.

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

cgit.h

@@ -183,6 +183,7 @@ struct cgit_config {
 	int summary_branches;
 	int summary_log;
 	int summary_tags;
+	struct cgit_filter *source_filter;
 };
 
 struct cgit_page {