Make --overrides more flexible

[epwalsh]

This proposed change impacts the behavior of --overrides in a potentially breaking way ⚠️

Overview

Previously the final configuration params read from a file were simply taken as the union over the original params in the file and the params specified in --overrides. With this, you can now use --overrides to completely replace any part of the original config.

For example, passing --overrides '{"model":{"type":"foo"}}' will completely replace the "model" part of the original config. However, when you just want to change a single field in the JSON structure without removing / replacing adjacent fields, you can still use the "dot" syntax. For example, --overrides '{"model.num_layers":3}' will only change the num_layers parameter to the "model" part of the config, leaving everything else unchanged.

Why is this useful?

With the old behavior it is impossible to remove any fields from the a config via --overrides. This becomes an issue when, for example, you want to change the type of your dataset_reader or model to a different type that takes completely different arguments.

cc @yakazimir