Ensured Sassdown can finish running even if options.assets returns null (optional property)
prevent overriding of unordered lists

`list-style-position: outside;` overwrites `list-style-type` of `ul` and `ol` elements.

with the result both `ul` and `ol` have 'disc' as `list-style-type`.
Merge pull request #52 from m5o/ordered-list

prevent overriding of unordered lists