Turning on legacy OCR engine mode

Whitelist and blacklist are not implemented in version 4.0 (issue) and user patterns do not work (issue). In this issue people recommend turning on legacy OEM using --oem 0 option flag. This option is not part of configs, but rather belong to the engine itself, like language.

Could we please enable more ocr options as arguments in tesseract::tesseract(), including oem to be able to temporarily switch to older version of the engine.