Add sass for sublists to remove negative top margin causing overlap.

[jatkinson1000]

Closes #33
I have left ul as having the original margin-top of -$margin-blocks/2 as I agree it is more aesthetic.
However, for any nested lists the margin-top is set to 0 to avoid overlap with the last item in the above layer.

e.g. when you have:

List of:

- 1
- 2
- 3
  - a
  - b
  - c