Allow variables in formulas

[bmschmidt]

For testing quantities, it would be nice to allow variables.

Currently, this code fails because Error in tree[[1]] : object of type 'symbol' is not subsettable. Looks like a parse error, not a namespace one, so should be an easy fix.

dist = 1.2
form = ~ "king" + dist * ("woman" - "man")
glove %>% closest_to(form)