get raw vectors for further processing in Word2Vec

e.g. clustering
Sorry for making one more iteration! The doc could be more concise, e.g.

~~~
Returns a map of words to their vector representations.
~~~

`for further processing (e.g. cluster)` is not necessary because we don't restrict how users will use it.

Usually, if `Returns ...` contains all the information, we don't need `@return`.

Another question I have is on the return type. We use `Array[Float]` to save space in computation. Shall we return `Map[String, Vector]` here, just to be consistent with others? @Ishiihara