Improve determination of element relevance to CSS counters (#705)

The previous commit was deficient in determining when an element
incremented the counter being parsed by the counters() function.
In the previous commit elements with a value less than 1 were
excluded from the output string. However, since elements can be set
to decrement counters we need to allow values less than 1. The code
now checks to see if the current element resets the counter being
processed.