MentionFrequency Primitive

As a user, I wish there was a NLP primitive that computed the frequency of mentions given a list of texts, as in https://stackoverflow.com/questions/49865756/extract-and-count-hashtags-from-a-dataframe/49865854#49865854. This is the mention analogue of #168.

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