allow passing param json via stdin in CLI

Sometimes it is preferable to specify parameters for processing on the fly. But the CLI module expects a filesystem path. It could be easily extended to allow - for stdin as well, by adding allow_dash=True to ocrd_tool_tool_parse_params, which is available since Click 6.0. This would enable on-the-fly specifications using here strings, e.g.:

ocrd-tesserocr-recognize -m mets.xml -p - <<<'{ "textequiv_level": "glyph", "model": "Fraktur" }'

[kba]

How about:

• Is it a readable filename?
  • yes: read it, parse it, continue
  • no: Does the string start with { (whitespace allowed)
    • yes: parse it
      • success: parse it, continue
      • failure: raise json parse exception
    • no: raise file not found exception

[mikegerber]

Looks good @kba 😃