asyncOutput option to allow output from async run

Introduces a new 'asyncOutput' flag that a user may wish to use in
conjunction with the existing 'async' flag, opting them into having the
Google App Engine SDK server sending all of its output even while other
tasks may be running.

This works by moving the /dev/null redirection out of the COMMAND_ASYNC
constant and making it templated via a new {redirect} placeholder.
Include asyncOutput in README documentation

Adds an item in the Options section explaining the new option for
controlling output.