consistent name for pos tags

[brendano]

the thing that is the sequence of POS tag strings.

get_phrases(postags=) but in the returned dict it's {'pos': [....]}

it's inconsistent because dict key 'pos' is how we do it in the corenlp wrapper and conll format converters. but it would be better to be consistent within this package.

[brendano]

by contrast, tokens is consistent (the sequence of word token strings).

get_phrases(tokens=) and the return dict is {'tokens': [...]}