Added support for 'line-no-highlight' class

Added the 'line-no-highlight' class to lines that are within an embed containing highlighting, making it possible to specifically style non-highlighted lines. The class doesn't apply to embeds that have no highlighting.
Changing the `elseif` to `else` negates the need for the class altogether, as the `line-no-highlight` class is only required when there *is* highlighting within the gist.