Add token scores to BaseStreamer.to_scipysparse()

[dkrasner]

When the token_col dictionary is updated it would be good to also update an overall count for each token, to later use for feature selection/filtering etc.

Perhaps the self.token_col_map should be self.token_col_count_map or there should be two separate attributes self.token_col_map and self.token_count_map - thoughts?