Add support for MENUITEMS and DISPLAY_PAGES_ON_MENU

With this commit, `.sidebar-nav` is renamed to `.sidebar-social`. Then a
new class `.sidebar-nav` is created and used for the menu items.  This
change more or less restores `.sidebar-nav` originally was in the
`hyde.css` file.

Since `DISPLAY_PAGES_ON_MENU` defaults to `True`, this commit will
modify the appearance of your site. To go back to the previous behavior,
add this to your pelicanconf.py:

    DISPLAY_PAGES_ON_MENU = False

References:
- <https://docs.getpelican.com/en/stable/settings.html#basic-settings>
- <https://docs.getpelican.com/en/stable/settings.html#MENUITEMS>

README.md

@@ -17,10 +17,13 @@ List of Pelican's settings that are supported by this theme. Refer to the
 for more details.
 
 - `FEED_*` and `*_FEED_*`
+- `DISPLAY_PAGES_ON_MENU`
 - `DISQUS_SITENAME`
 - `GOOGLE_ANALYTICS`
 - `GA_COOKIE_DOMAIN`
 - `SITESUBTITLE`
+- `MENUITEMS`
+- `SOCIAL`
 
 Additional settings: