BUG: Allow concat to take string axis names
Continued in #14416 

---
- [x] closes #14369 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

This uses [`_get_axis_number`](https://github.com/pydata/pandas/blob/master/pandas/core/generic.py#L327-L338) to convert a string `axis` parameter to an integer.  This will allow the `concat` method to use any other aliases given to dataframes axes in the future while not disrupting other uses of `axis` in the `concat` method.  If this pattern works well, it could be used for the other [merge functions](https://github.com/pydata/pandas/blob/master/pandas/tools/merge.py) also.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14389?src=pr) is 85.26% (diff: 100%)

> Merging [#14389](https://codecov.io/gh/pydata/pandas/pull/14389?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14389   diff @@
==========================================
  Files           140        140          
  Lines         50634      50636     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43173      43174     +1   
- Misses         7461       7462     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [d98e982...3f08b07](https://codecov.io/gh/pydata/pandas/compare/d98e982ecb0f6b7f3478a63caac34e0be238895c...3f08b074172760b0f51d0273662621ce451c0f11?src=pr)
 Thanks for the feedback!
 By the way, I can squash all of these commits into one once we get everything finalized.
 > By the way, I can squash all of these commits into one once we get everything finalized.

Squashing is not really needed anymore, we squash when merging anyway.
 My most recent commit ( brandonmburroughs/pandas@24eb09d ) hasn't shown up. It seems like the appveyor tests never responded back to Github that it finished. Maybe that has something to do with it?  Any ideas on how to fix this and get the latest commit to show up?
 Hmm, not sure why. We moved from pydata org to pandas-dev org, but I would not expect that to cause this issue. But it's maybe a reason the appveyor status is not correct (as it in fact already completed the build)
