[text.CaseUtils] adding support for snake_case and kebab-case

[Bryan Rickman]

I recently used the text.CaseUtils for converting to camelCase and
PascalCase. I also needed to convert to snake_case and kebab-case, and
ended up writing my own code for that (I wasn't really a big fan of other
utility options out there). Would it be well received if I submitted a PR
for adding that support to CaseUtils? If so, any recommendations on what to
include or not include?