Print dump stats at the end

[dvarrazzo]

The comments in the output file contain size info: at the end of a dump I add sometimes:

echo "The biggest tables in the dump:" >&2
grep "bytes written" data.sql | cut -c 4- | sort -nr | head >&2

this could be done automatically.

Note that it only works if -o is specified, otherwise the file is not seekable and psycopg doesn't tell us how much it copied out (although it might... mmm...)