HashtagFrequency Primitive

[sbadithe]

As a user, I wish there was a NLP primitive that computed the frequency of hashtags given a list of texts, as in https://stackoverflow.com/questions/49865756/extract-and-count-hashtags-from-a-dataframe/49865854#49865854.

#139 is similar, but computes the hashtag count per tweet. This primitive would compute the frequency of each tag given a list of inputs.