Potentially unneeded memory alloc in `StopwordCount`

[sbadithe]

I don't think we need to create a list in this comprehension:

nlp_primitives/nlp_primitives/stopword_count.py

Line 42 in 11837a5

count = len([word for word in words if word.lower() in swords])

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.