Drop .row_col argument from kmeans_tidy function

[spsanderson]

Is your feature request related to a problem? Please describe.
Since the kmeans_tidy function from #40 uses the input from the kmeans_user_item_tbl() function, there is no need to provide .row_col instead just slice off column one internally as uit_tbl[1] which will require less user input.

Describe the solution you'd like
Internally make the column equal to uit_tbl[1]

uit_col <- uit_tbl[1]
OR
uit_col <- uit_tbl %>% dplyr::select(1)