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.
+1 to using the 4-letter extension `.ecsv` (no-one uses MS-DOS any more and I think the 3-letter limit died with it?) and auto-registering the extension.