ERR: qcut uniquess checking (try 2)
 - [x] closes #7751
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Add option to drop non-unique bins.
My second try on https://github.com/pandas-dev/pandas/pull/14455 -- I wasn't able to rebase this one. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15000?src=pr) is 84.78% (diff: 87.50%)
> Merging [#15000](https://codecov.io/gh/pandas-dev/pandas/pull/15000?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #15000   diff @@
==========================================
  Files           145        145          
  Lines         51090      51095     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43315      43319     +4   
- Misses         7775       7776     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7f0eefc...698b4ec](https://codecov.io/gh/pandas-dev/pandas/compare/7f0eefc2f81d6b34f252a5da38d31e515c295106...698b4ecf284283604673ee393bf006de9d38983d?src=pr) pls add a whatsnew in 0.20.0 (other enhancements) @ashishsingal1 For future reference, it is not needed to start a new PR, you can always clean up locally and just force push to the same branch in your fork, and this PR will be updated automatically. In fact, we rather much prefer that instead of opening a new PR, to keep discussions gathered at one place. If you need help with those git aspects, don't hesitate to ask! @jreback @jorisvandenbossche anything left for this PR on my side? thanks!