syntax-extension: add "priv" field.

+ And pointer to a free function as well.

src/cmark_extension_api.h

@@ -97,6 +97,10 @@ typedef struct cmark_plugin cmark_plugin;
  *
  * Finally, the extension should return NULL if its scan didn't
  * match its syntax rules.
+ *
+ * The extension can store whatever private data it might need
+ * with 'cmark_syntax_extension_set_private',
+ * and optionally define a free function for this data.
  */
 typedef struct cmark_syntax_extension cmark_syntax_extension;
 
@@ -249,6 +253,13 @@ CMARK_EXPORT
 void cmark_syntax_extension_set_special_inline_chars(cmark_syntax_extension *extension,
                                                      cmark_llist *special_chars);
 
+/** See the documentation for 'cmark_syntax_extension'
+ */
+CMARK_EXPORT
+void cmark_syntax_extension_set_private(cmark_syntax_extension *extension,
+                                        void *priv,
+                                        cmark_free_func free_func);
+
 /** Return the index of the line currently being parsed, starting with 1.
  */
 CMARK_EXPORT

src/syntax_extension.c

@@ -5,6 +5,10 @@
 #include "buffer.h"
 
 void cmark_syntax_extension_free(cmark_syntax_extension *extension) {
+  if (extension->free_function && extension->priv) {
+    extension->free_function(extension->priv);
+  }
+
   cmark_llist_free(extension->special_inline_chars);
   free(extension->name);
   free(extension);
@@ -42,3 +46,10 @@ void cmark_syntax_extension_set_special_inline_chars(cmark_syntax_extension *ext
                                                      cmark_llist *special_chars) {
   extension->special_inline_chars = special_chars;
 }
+
+void cmark_syntax_extension_set_private(cmark_syntax_extension *extension,
+                                        void *priv,
+                                        cmark_free_func free_func) {
+  extension->priv = priv;
+  extension->free_function = free_func;
+}

src/syntax_extension.h

@@ -11,6 +11,8 @@ struct cmark_syntax_extension {
   cmark_inline_from_delim_func    insert_inline_from_delim;
   cmark_llist                   * special_inline_chars;
   char                          * name;
+  void                          * priv;
+  cmark_free_func                 free_function;
 };
 
 #endif