stbt batch run: Add "-o" to specify output directory

This makes it easier to run "stbt batch run" as the target of an ssh
command. For example:

    ssh server.example.com stbt batch run -o /var/stbt-results/ ...

I didn't want to use "-C" a la make/tar/git because one day we might
want to use that to specify the input directory to save typing long test
pathnames. (`stbt batch run -C /long/path/to/tests test1.py test2.py`
instead of: `stbt batch run /long/path/to/tests/test1.py
/long/path/to/tests/test2.py`.)

"stbt batch run" doesn't output a single file so I think it's safe to
use "-o" for the output directory command-line option.

This doesn't support spaces in the output directory name, due to a bug
in "stbt batch report".