Fix #199: adding option -o to the converters

[andreasabel]

With added option -o, the interfaces look like this:

$ yaml2json -h

yaml2json version 0.11.6.0 https://github.com/snoyberg/yaml/
Convert YAML to JSON.

Usage: yaml2json [IN] [-o|--output OUT]

Available options:
  -h,--help                Show this help text
  -V,--version             Show version info.
  --numeric-version        Show just version number.
  IN                       The input file containing the YAML document; use '-'
                           for stdin (default).
  -o,--output OUT          The file to hold the produced JSON document; use '-'
                           for stdout (default).

$ json2yaml -h

json2yaml version 0.11.6.0 https://github.com/snoyberg/yaml/
Convert JSON to YAML.

Usage: json2yaml [IN] [-o|--output OUT]

Available options:
  -h,--help                Show this help text
  -V,--version             Show version info.
  --numeric-version        Show just version number.
  IN                       The input file containing the JSON document; use '-'
                           for stdin (default).
  OUT                      The file to hold the produced YAML document; use '-'
                           for stdout (default).

The old call pattern 'json2yaml IN OUT' is also accepted, but deprecated.

[snoyberg]

Thanks!

[andreasabel]

Thanks for the best responsiveness to PRs ever!

[snoyberg]

That’s high praise, thank you!