renamed and re-ordered method

seems more appropriate this way

markdown_checklist/extension.py

@@ -23,13 +23,13 @@ class ChecklistPostprocessor(Postprocessor):
 
     pattern = re.compile(r'<li>\[([ Xx])\]')
 
-    def checklister(self, match):
-        state = match.group(1);
-        checked = ' checked' if state != ' ' else ''
-        return '<li><input type="checkbox" disabled%s>' % checked
-
     def run(self, html):
-        html = re.sub(self.pattern, self.checklister, html)
+        html = re.sub(self.pattern, self._convert_checkbox, html)
         before = '<ul>\n<li><input type="checkbox"'
         after = before.replace('<ul>', '<ul class="checklist">')
         return html.replace(before, after)
+
+    def _convert_checkbox(self, match):
+        state = match.group(1)
+        checked = ' checked' if state != ' ' else ''
+        return '<li><input type="checkbox" disabled%s>' % checked