Vectorize the api method functions

[yl3394]

The output token (annotate_text()$tokens)is a table for each string. But when I try to do text-mining on a list of text messages, I have to append all per-string token tables one by one. So I am wondering is it possible to make the function can be ran on a list of string and return one single table at once, similar to the functionality of tidytext::unnest_tokens().