Better generator integration/example

Creating a `data.npz` file may result in too large a file for some projects. At that point it may be better to read the training data in with a generator (example: [Keras flow_from_directory](https://keras.io/preprocessing/image/)). The easiest way to do this now would be to skip the final step (`label-maker package`) and read directly from the `labels.npz` and the `tiles` directory. We should create either (1) an example showing how to do this or (2) additional scripts which perform this for the user depending upon configuration (or a combination of the two).