add tasklist extension (commonmark#94)

* add tasklist extension

* recognize “+” as list marker

* use long type to avoid warning in MSVC environment

* recognize ordered list

> Two list items are of the same type if they begin with a list marker of the same type. Two list markers are of the same type if (a) they are bullet list markers using the same character (-, +, or *) or (b) they are ordered list numbers with the same delimiter (either . or )).

( https://github.github.com/gfm/#task-list-items-extension- )

* add preliminary tests

* fix regular extension tests

* fix tasklist nesting

Author: Watson1978

extensions/CMakeLists.txt

@@ -10,6 +10,7 @@ set(LIBRARY_SOURCES
     ext_scanners.c
     ext_scanners.re
     ext_scanners.h
+    tasklist.c
    )
 
 include_directories(

extensions/core-extensions.c

@@ -3,6 +3,7 @@
 #include "strikethrough.h"
 #include "table.h"
 #include "tagfilter.h"
+#include "tasklist.h"
 #include "registry.h"
 #include "plugin.h"
 
@@ -12,6 +13,7 @@ static int core_extensions_registration(cmark_plugin *plugin) {
                                          create_strikethrough_extension());
   cmark_plugin_register_syntax_extension(plugin, create_autolink_extension());
   cmark_plugin_register_syntax_extension(plugin, create_tagfilter_extension());
+  cmark_plugin_register_syntax_extension(plugin, create_tasklist_extension());
   return 1;
 }