Allow CheckpointManager to best model only

[mjurkus]

Currently, CheckpointManager saves all checkpoints. I would like to have an option to save only when the model improved. Basically same as tf.keras.callbacks.ModelCheckpoint

[nglehuy]

@mjurkus Updated to support training with keras built-in fit, which means it supports keras callbacks like ModelCheckpoint. Checkout the #118 for more detail 😄

[mjurkus]

Yes, I saw the release :) Great news and thank you!