Pytorch-lightning: Add an easy way to manipulate progressbar metrics

[mdmmn378]

Until now, the progress bar only shows a limited number of parameters. It'd be very helpful if we could directly manipulate internal tqdm metrics.

[williamFalcon]

that's already possible:
https://pytorch-lightning.readthedocs.io/en/latest/LightningModule/RequiredTrainerInterface/#training_step

    output = {
        'loss': loss, # required
        'prog': {'tng_loss': loss, 'batch_nb': batch_nb} # optional
    }