add stream_command(cmd, stdout) to dbsake.util

- This allows for opening a subprocess to stream
  data through. This is intended for compression commands
  that accept data on stdin and output data on stdout to
  perform some sort of transformation on the data.
prettify the output of deferred fast-alter-table
add filter_command option and filter all output files through it

- this adds split-mysqldump --filter-command to allow customizing
  the compression (or transformation) command used when creating
  output files.  By default this is set to gzip -1 to perform
  fast gzip compression on the output files.
mkdir_safe now returns True if a directory was created

- this is to allow detection when a directory is actually
  created and a command can inform the end user
make mysqldumpsplit.defer use logging rather than print
log database/table/view counts at the end of the split process