--output -> --output-file/-o & --output-format/-O, refactor

Use two options and less surprising behaviour. Notes:
* a recognised suffix on the output file name can set the format
* but --output-format always wins
* -o - means stdout
* currently these are command-specific options on balance, print, register
* later, --output-file should become a global option
* and --output-format should be available at least on all commands which
  support multiple formats. Each command may support a different set of
  output formats, which should be listed in its command line help.