do not support get sentence vector

[xiaomj]

[Codekun]

Hi, I had the same problem using this package, and I found the C++ code for getting the sentence vector.
https://github.com/facebookresearch/fastText/blob/a20c0d27cd0ee88a25ea0433b7f03038cd728459/src/fasttext.cc#L490
I translated the code to a java version and got the same result.