support markdown code

src/skill_tree/exercises/markdown.py

@@ -192,9 +192,13 @@ def paragraph(state):
 
 @Parsec
 def code(state):
-    left = attempt(string("```")).then(many(attempt(inline))).over(string("\n"))
+    side = choice(attempt(string("````")), string("```"))
+    left = many(attempt(inline)).over(string("\n"))
+
+    side_token = side(state)
     language = ''.join(left(state))
-    right = attempt(string("\n```").over(choice(attempt(string("\n")), eof)))
+
+    right = attempt(string("\n"+side_token).over(choice(attempt(string("\n")), eof)))
     buffer = ""
     while True:
         try:

src/skill_tree/tree.py

@@ -367,7 +367,7 @@ class TreeWalker:
                 export.append(mfile)
                 flag = True
                 config["export"] = export
-            with open(md_file, encoding="utf-8") as efile:
+            with open(md_file, "r", encoding="utf-8") as efile:
                 data = efile.read()
                 state = BasicState(data)
                 try: