Allow multiple configurations of polarity

For instance:

```r
polarity(data_dictionary_LSD2015) <- 
  list(simple = list(pos = "negative", neg = "negative"),
       expanded = list(pos = c("positive", "neg_negative"), neg = c("negative", "neg_positive")))
```
then the output would be:
```
  doc_id  simple  expanded
      d1     1.3       1.6
      d2    -2.1      -2.3
```
instead of a single comment called `sentiment`.
