feat: extend markdown to support macros

With this feature, the exercises can be added to the lesson as:

    {{ Exercise("two-circles") }}

This also added fenced_code extension that allows adding id and classes
to code blocks.

This uses Python-Markdown library instead of Markdown2 that is used
everywhere in Frappe. The Python-Markdown is more easily extensible than
Markdown2.

Issue #115

requirements.txt

@@ -1,2 +1,5 @@
 frappe
 websocket_client
+markdown
+beautifulsoup4
+lxml