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.
Yes, that was intended.  Not sure of the best numbers except that the "test for table length" should be noticeably bigger than the test initial conversion length.