Usage (ranges are space separated):

{% highlight js 1,4-6 %}
One range
Adds `highlight-line-active` to lines 1,4,5,6

{% highlight js 3-4 -1 %}
Two ranges (add/remove), remove is N/A
Adds `highlight-line-add` to lines 3,4

{% highlight js -1 3-4 %}
Two ranges (add/remove), add is N/A
Adds `highlight-line-remove` to lines 3,4

{% highlight js 3-4 5,8-10 %}
Two ranges, both are used
Adds `highlight-line-add` to lines 3-4
Adds `highlight-line-remove` to lines 5,8,9,10

Fixes #5

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