txtfilereader读插件关于分隔符只支持一位长度，能否支持多位

txtfilereader读插件关于fieldDelimiter参数能否支持多位分隔符情况，如下代码能否修改：
String delimiterInStr = this.originConfig.getString(com.alibaba.datax.plugin.unstructuredstorage.reader.Key.FIELD_DELIMITER);
// warn: if have, length must be one
if (null != delimiterInStr && 1 != delimiterInStr.length()) {
throw DataXException.asDataXException(
UnstructuredStorageReaderErrorCode.ILLEGAL_VALUE,
String.format("仅仅支持单字符切分, 您配置的切分为 : [%s]",
delimiterInStr));
}

Start in the txtfilereader plugin at the handling of Key.FIELD_DELIMITER, beginning with the shown originConfig validation. Trace how the delimiter is consumed after validation and run the relevant reader tests if available. Done means multi-character delimiters are accepted and existing single-character behavior remains intact.

Written by the indexing model from the issue text.