Add useEncoding to SimpleExcelReader

[aminevg]

This PR adds a useEncoding method to SimpleExcelReader to allow use of custom encodings (SHIFT_JIS, EUC_JP, etc).

Currently, opening files in encodings other than UTF-8 can lead to garbled output. This allows us to set the exact encoding we want the data to be in.

This also enables us to check what encoding our data is in. An example:

function check_csv_encoding(string $path, string $encoding): bool
{
    try {
        SimpleExcelReader::create($path)->useEncoding($encoding)->getHeaders();

        return true;
    } catch (\OpenSpout\Common\Exception\EncodingConversionException $e) {
        return false;
    }
}

I am not sure if this should be something this library should handle, but if needed, I can make a PR for that as well.

[freekmurze]

Thanks!