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.
Did you deliberately test for table length 10,000 then try converting only the first 1,000? Nothing is wrong with that, just wasn't sure it was what you intended.