Add generic filter/plugin infrastructure

The functions cgit_open_filter() and cgit_close_filter() can be used to
execute filters on the output stream from cgit.

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

cgit.h

@@ -129,6 +129,15 @@ struct cgit_query {
 	int showmsg;
 };
 
+struct cgit_filter {
+	char *cmd;
+	char **argv;
+	int old_stdout;
+	int pipe_fh[2];
+	int pid;
+	int exitstatus;
+};
+
 struct cgit_config {
 	char *agefile;
 	char *cache_root;
@@ -248,5 +257,8 @@ extern const char *cgit_repobasename(const char *reponame);
 
 extern int cgit_parse_snapshots_mask(const char *str);
 
+extern int cgit_open_filter(struct cgit_filter *filter);
+extern int cgit_close_filter(struct cgit_filter *filter);
+
 
 #endif /* CGIT_H */