Allow to remove @ and # from word frequency counts

[ErikBorra]

Currently word counts make a distinction between [word], [@word], and [#word]. The idea is that you'd want to be able to make a distinction and that @mentions and #hashtags are different types of signifiers. Sometimes, however, you might just want to look at words and treat [word, @word, #word] as the same thing. We should provide a checkbox / option for the word frequency module to remove @ and # from words.