Rename "model.go" to "predictor.go".
Replace "Model" with "Predictor".
Fix: Encode the context for prediction with "(*Predictor).wordTrie".
Encode the context words and find the "prefix" node in "(*Predictor).ngramTrie.
Encode the context with the prefix of a word to be predicted.
TODO: Generate top-K completetions for the given context and prefix.
Merge branch 'feature/predictor' into master