Return token_strings in Client.tokenize() when offline=True

[yifanmai]

Fixes #493

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[abdullahkady]

Thanks @yifanmai for the contribution!

We intentionally skipped this when we introduced offline local tokenization; since we were optimizing for time. However, let me check how much overhead this adds (for bigger inputs) and if it's negligible we can add it.

[yifanmai]

Noted, thanks!