BUG: Datetime64Formatter not respecting ``formatter``
- [x] closes #10690
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

the Datetime64Formatter class did not accept a `formatter` argument, so custom formatters passed in through `df.to_string` or `df.to_html` were silently ignored. 
