Proper display of nested dict in param.

[lkhphuc]

Support proper display of nested params file since many application provided a nested dictionary of parameters.
For example:

Params
data._target_:             project.data.MNISTDataModule
data.batch_size:         32
model.base:                resnet18
model.pretrain:          true
model.optim.lr:          1e-3

[bfirsh]

Yes! Thanks for the suggestion. A simple solution would be to just format the JSON with newlines and indenting. Another alternative would be to have a YAML-like value, which might be easier to read and see the structure of.