Support digits as tokens

[boblannon]

Is your feature request related to a problem? Please describe. I often have column names in source data that look like:

CatalogGroup1
CatalogGroup2
CatalogGroup3
CatalogGroup4
CatalogGroup5

The depascalize transform produces, eg: catalog_group1.

Describe the solution you'd like

It would be ideal to produce that final digit as its own token in the result variable, eg catalog_group_1.