How to add new indexes to my net?


## Summary
I will jump to the example and explain later.

### Basic example
Imagine a network like that:
```js
let data = [
["Hello", " ", "World", "!"],
["Lorem", " ", "Ipsum"]
]

let net = new Brain.recurrent.LSTM();

//pre-training
net.train(data);

//post-training
let runData = ["Hello", " ", "Dolor", "!"];

net.train(data.concat(runData)); //My attempt to include "Dolor" to the indexes.

console.log(net.run(runData));
```
It should throw:
```js
Error: unrecognized character "Dolor"
```

### Motivation
My point is to provide a way to add indexes to the model and then update my saved model.

Sorry if this already exists.