Add new token classification example
# What does this PR do?

This PR adds a new example script leveraging the dataset library, the offsets from the fast tokenizers of the tokenizers library and the Trainer API.
Hi @sgugger , thanks for these new features :hugs: 

I have a some comments:

* Only training and validation files can be passed via command line argument. The `do_predict` method is already supported, so a new argument for specifiying the test file would be great to have!
* A new version of the `run.sh` script would also be a great feature to start fine-tuning just be executing a shell script. The current `run.sh` script should also get an "_old" suffix then :)