add totext method that can fetch the content directly

[cxx199921110]

i use below method currently to fetch the node content：

==================
private String toText(String content, TSNode node) {
byte[] bytes = content.getBytes(StandardCharsets.UTF_8);
int startByte = node.getStartByte();
int endByte = node.getEndByte();
byte[] nodeBytes = Arrays.copyOfRange(bytes, startByte, endByte);
return new String(nodeBytes, StandardCharsets.UTF_8);
}

hope to add add totext method like python that can fetch the content directly