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.
Why not have `ascii.ecsv` auto-identify? Understand there may not be time for that now and/or it isn't clear yet what the default extension should be.