Add support for Keras ImageDataGenerator

[jakeret]

Keras provides a nice API for loading and also transforming training and validation data. Maybe with a few tweaks this could be supported by tf_unet.

Example:

train_datagen = image.ImageDataGenerator(
    preprocessing_function=preprocess_input,
    rotation_range=30,
    width_shift_range=0.2,
    height_shift_range=0.2,
    shear_range=0.2,
    zoom_range=0.2,
    horizontal_flip=True,
    vertical_flip=True
)

train_generator = train_datagen.flow_from_directory(
  './data/food-101/train/',
  target_size=target_size,
  batch_size=32,
)

Afterwards the generator can be iterated indefinitely e.g. data, mask = next(train_generator).