Move sidebar sublist into nested lists. Fixes #47.
Straightens out spacing for nested list-group-items.

The list-group class has a fairly extensive margin at the bottom. If we nest
list groups within list group items, this and the padding of list group items
result in excessive spacing between one nested group to the next. This fixes
that in regard to #47.
Adds ids to sublists in sidebar & example for use.

This allows one to control the style of sublists in the sidebar individually.
The example adds a small amount of spacing between icon and icon label in the
SOCIAL sublist. Thanks to @DandyDev for the suggestion.