Add new layout and theming.

Keep tweaking site layout.
Add support for "tips" pages.

Add a new "tips" collection and _tips subfolder for standalone
tips/tricks/techniques.
Set content type for /tips/.

Set the default content type for the /tips/ folder to text/html. Jekyll
won't copy .htaccess from the special "_tips" collection folder, so we
create a placeholder "tips" folder (no underscore) containing the
.htaccess. _site/tips will contain the combined output of both folders.
Update clean URLs plugin to handle index.html.

Update clean URLs plugin to output an "index.html" file (with extension)
for collection folders (like _tips/tips).