add optional downCaseTail arg to capitalize

- calling `capitalize` without the optional `downCaseTail` has the same behavior as `capitalize` pre version 1.4.0
- calling `capitalize` with `downCaseTail=true` will downcase the non-first-characters in the input string
   - `capitalize("wHoOA!")==="WHoOA!"`
   - `capitalize("wHoOA!", true)==="Whooa!"`