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

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- [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.
