Add extra/optional Markdown dependency to setup.py

- Modify documentation to use the extra dependency in order to install
the recommended markdown package version.

docs/quickstart.rst

@@ -11,7 +11,7 @@ Install Pelican (and optionally Markdown if you intend to use it) on Python
 2.7.x or Python 3.5+ by running the following command in your preferred
 terminal, prefixing with ``sudo`` if permissions warrant::
 
-    pip install pelican markdown
+    pip install pelican[Markdown]
 
 Create a project
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