[Feature Request] Allow to pass strict=False when loading model

[araffin]

In SB2, for load_parameters, we had a exact_match parameter to allow key mismatch when loading.
It would be nice to have that, we have to exclude optimizers state dict though.
This feature allows for instance to load models with same architecture but different names (recently there was a rename with #92 that broke previously saved policy, with strict=False we can still load the old models that are still valid).

We also had a custom_objects parameter to replace parameters on the fly, not sure if it is still needed... @Miffyli could you comment on that?

[Miffyli]

I will include this in the part of reviewing over saving/loading code, currently playing around with A2C/PPO discrete performance.

I need to check if still need custom_objects parameter. I had to use it couple of times in the past to lazily skip loading some only-training-related things that were not required during evaluation.