Checks for left_index and right_index merge parameters
Hi,

I just committed an error when I was doing an analysis using pandas and this motivated me to implement two checks which in my opinion are necessary. 

I was trying to perform a merge and I confused the parameters "left_on" and "right_on" for "left_index" and "right_index". I ran the code and it did not raised me any error. It produced a table which seem to be fine in terms of shape. I suspect that what happened is that the tables got merged by the index of the data frame. I think it would be a great idea to check if right_index and left_index are of type bool, if not, raise an error. This way we will avoid that more people got the same error as mine :D.

Tests passed 

Thanks!

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14434?src=pr) is 85.25% (diff: 100%)

> Merging [#14434](https://codecov.io/gh/pandas-dev/pandas/pull/14434?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14434   diff @@
==========================================
  Files           140        140          
  Lines         50631      50635     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43166      43171     +5   
+ Misses         7465       7464     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [c31ea34...e18b7c9](https://codecov.io/gh/pandas-dev/pandas/compare/c31ea341747ac40d83e64d00bac179548e4b716e...e18b7c92cd62139cb6984d5af638b9a0554ddb26?src=pr)
 Updated!
 Is there still any change to do? Thanks!
 pls add a whatsnew entry in 0.19.1. ping on green.
 Done!
 @jreback green
 @ivallesp No matter, @jreback already merged and addressed those comments!
 @jorisvandenbossche was your 2nd one the comment reference in the tests? (decided not a big deal)
 yep, but indeed not a big deal
