VIS: Allow 'C0'-like plotting for plotting colors #15516
 - [x] closes #15516 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Defined special case for 'CN' cases where N is 0 to 9. There may be custom color cycles that can also be parsed as a single color so the ValueError message is left intact (unless this is false).

```
running: pytest --skip-slow --skip-network pandas
============================= test session starts ==============================
platform linux2 -- Python 2.7.13, pytest-3.0.7, py-1.4.33, pluggy-0.4.0
rootdir: /home/sfoo/pandas, inifile: setup.cfg
collected 11894 items / 3 skipped 

10234 passed, 1586 skipped, 77 xfailed, 2084 pytest-warnings in 486.80 seconds 
```