Support table metadata in io.ascii

This is an initial feature request to more fully support table metadata in `io.ascii`.  This was  discussed in #664 and #659.

This issue is meant to address just reading and writing metadata into an ASCII-formatted table.

There is the related issue of providing a framework for manipulating metadata beyond the current concept of a simple ordered dict.  This has a much larger scope and potentially touches `nddata`, `io.fits` and others. 

cc: @kbarbary