liquid_tags: make it pelican 3.7 compatible

liquid_tags/liquid_tags.py

@@ -3,18 +3,22 @@ from .mdx_liquid_tags import LiquidTags, LT_CONFIG
 
 
 def addLiquidTags(gen):
-    if not gen.settings.get('MD_EXTENSIONS'):
+    if not gen.settings.get('MARKDOWN'):
         from pelican.settings import DEFAULT_CONFIG
-        gen.settings['MD_EXTENSIONS'] = DEFAULT_CONFIG['MD_EXTENSIONS']
+        gen.settings['MARKDOWN'] = DEFAULT_CONFIG['MARKDOWN']
 
-    if LiquidTags not in gen.settings['MD_EXTENSIONS']:
+    if LiquidTags not in gen.settings['MARKDOWN']:
         configs = dict()
         for key,value in LT_CONFIG.items():
             configs[key]=value
         for key,value in gen.settings.items():
             if key in LT_CONFIG:
                 configs[key]=value
-        gen.settings['MD_EXTENSIONS'].append(LiquidTags(configs))
+        gen.settings['MARKDOWN'].setdefault(
+            'extensions', []
+        ).append(
+            LiquidTags(configs)
+        )
 
 
 def register():