Implement support for the ECSV format proposed in APE6

APE6 proposes a new standard Data-table Text Interchange Format for storing data tables in a text-only format.  This PR provides a demonstration implementation of that in `astropy.io.ascii`.  This is by no means complete and should not be merged.
Can you put this instead so that the example also works in Python 3?
```
from astropy.extern.six.moves import StringIO
```