load_dataset doesn't include `features` in its hash
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It looks like the function `load_dataset` does not include what's passed in the `features` argument when creating a hash for a given  dataset. As a result, if a user includes new features from an already downloaded dataset, those are ignored.

Example: some models on the hub have a different ordering for the labels than what `datasets` uses for MNLI so I'd like to do something along the lines of:
```
dataset = load_dataset("glue", "mnli")
features = dataset["train"].features
features["label"] = ClassLabel(names = ['entailment', 'contradiction', 'neutral'])  # new label order
dataset = load_dataset("glue", "mnli", features=features)
```
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