cut() a variable with missing values

[wibeasley]

@genevamarshall & @thomasnwilson, in the past three days, you both have asked about a function like this: https://stackoverflow.com/questions/50882620/cut-a-variable-with-missing-values/50882621

If there's not a better response, I'll test & add this new to this package.

Note to self, add additional cut() parameters like

• [x] include.lowest and
• [x] right.

[wibeasley]

Closed in June. The include.lowest and right are passed to base::cut() with ....