Lda bag of words export model

I use LDA transformation from example

```
var pipeline =
ml.Transforms.Text.ProduceWordBags(review).
Append(ml.Transforms.Text.LatentDirichletAllocation(review, ldaFeatures, numberOfTopics: 3));

var transformer = pipeline.Fit(trainData);
var transformed_data = transformer.Transform(trainData);

```

Now i try to visualize data with [pyLDAvis](https://github.com/bmabey/pyLDAvis)

For this task i need phi matrix, theta matrix, vocabulary, term_frequency.

It is possible to get theta matrix using documents transform,
It is possible to get phi matrix using, with SingleBox.GetModel internal method (using Reflection)
It din't managed to get the vocabulary.

For a moment it is to hard to export LDA related parameters from ml pipline.

If would be nice to be able to export complete set of related LDA parameters