Potentially unneeded memory alloc in `StopwordCount`

[sbadithe]

I don't think we need to create a list in this comprehension: https://github.com/alteryx/nlp_primitives/blob/11837a50de79fd05e067f58075100f39f639e563/nlp_primitives/stopword_count.py#L42

Instead, I think we can just iterate over words and keep a count variable. For very large strings this would save the time and memory needed to alloc the list.