Avoid repeated overwrites of config['extra_*']

Fixes #133
Well, it makes it so that I can't build docs without having some extra CSS and JS. Notably, it fails if you try to build MkDocs' documentation, since it doesn't have any extra Javascript. Here's the backtrace:

```
Traceback (most recent call last):
  File "/home/jim/src/mkdocs/env/bin/mkdocs", line 9, in <module>
    load_entry_point('mkdocs==0.9', 'console_scripts', 'mkdocs')()
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/mkdocs-0.9-py2.7.egg/mkdocs/main.py", line 52, in run_main
    main(cmd, args=sys.argv[2:], options=dict(opts))
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/mkdocs-0.9-py2.7.egg/mkdocs/main.py", line 29, in main
    serve(config, options=options)
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/mkdocs-0.9-py2.7.egg/mkdocs/serve.py", line 78, in serve
    build(config, live_server=True)
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/mkdocs-0.9-py2.7.egg/mkdocs/build.py", line 205, in build
    build_pages(config)
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/mkdocs-0.9-py2.7.egg/mkdocs/build.py", line 190, in build_pages
    output_content = template.render(context)
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/Jinja2-2.7.1-py2.7.egg/jinja2/environment.py", line 969, in render
    return self.environment.handle_exception(exc_info, True)
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/Jinja2-2.7.1-py2.7.egg/jinja2/environment.py", line 742, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/jim/src/mkdocs/env/local/lib/python2.7/site-packages/mkdocs-0.9-py2.7.egg/mkdocs/themes/mkdocs/base.html", line 63, in top-level template code
    {% for path in extra_javascript %}<script src="{{ path }}"></script>{% endfor %}
TypeError: 'NoneType' object is not iterable
```