ENH: Added 'direction' parameter to merge_asof() (#14887)
 - [x] closes #14887
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry
lgtm (minor doc-comment). already green (appeveyor seems stuck, working on that indepdendtly). so ping when pushed.

also just verify that perf is the same. ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15129?src=pr) is 86.26% (diff: 88.88%)
> Merging [#15129](https://codecov.io/gh/pandas-dev/pandas/pull/15129?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **0.72%**

```diff
@@             master     #15129   diff @@
==========================================
  Files           145        145           
  Lines         51274      54303   +3029   
  Methods           0          0           
  Messages          0          0           
  Branches          0          0           
==========================================
+ Hits          43857      46844   +2987   
- Misses         7417       7459     +42   
  Partials          0          0           
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [5a0eb5a...879c9f0](https://codecov.io/gh/pandas-dev/pandas/compare/5a0eb5ad4cae2818c01f46b79121b992f98eeae4...879c9f02d6467e7cdd64e2333b2eb2a3cd2a8ba4?src=pr) @jreback I've pushed the bullet-pointed description. ASV came-out the same, which I would expect since the "backward" direction is the same code. thanks!