Import default markdown extensions from settings instead of readers

Closes getpelican/pelican-plugins#69

Due to change in pelican code, `import EXTENSIONS` was no longer
working.

liquid_tags/liquid_tags.py

@@ -1,15 +1,16 @@
 from pelican import signals
 from mdx_liquid_tags import LiquidTags
-from pelican.readers import EXTENSIONS
+
 
 def addLiquidTags(gen):
     if not gen.settings.get('MD_EXTENSIONS'):
-        MDReader = EXTENSIONS['markdown']
-        gen.settings['MD_EXTENSIONS'] = MDReader.default_extensions
-    
+        from pelican.settings import DEFAULT_CONFIG
+        gen.settings['MD_EXTENSIONS'] = DEFAULT_CONFIG['MD_EXTENSIONS']
+
     if LiquidTags not in gen.settings['MD_EXTENSIONS']:
         configs = dict(settings=gen.settings)
         gen.settings['MD_EXTENSIONS'].append(LiquidTags(configs))
- 
+
+
 def register():
     signals.initialized.connect(addLiquidTags)