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. 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13567?src=pr) is **84.34%**

> Merging [#13567](https://codecov.io/gh/pydata/pandas/pull/13567?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.03%**

``` diff
@@             master     #13567   diff @@
==========================================
  Files           138        138          
  Lines         51155      51135    -20   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43130      43132     +2   
+ Misses         8025       8003    -22   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [2655dae...3146ded](https://codecov.io/gh/pydata/pandas/compare/2655daef1b7346feabd00d4d40910a80386d0812...3146ded17a83c691c75ff15dd0b369e723b77ffe?src=pr)
 lgtm. 
 @haleemur Merged, thanks a lot!

(I moved the whatsnew note to 0.19.0, as master is now targeted for that)
