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, I wasn't sure what the extension should be.  There was some previous discussion about a 3-letter version vs. 4-letter, so I thought that for now we can leave it without auto-identify.