Enable added tokens
Currently, the only way to manage adding `AddedToken`s to a tokenizer is via the `tokenizer.add_special_tokens` or `tokenizer.add_tokens` methods; it should also be enabled from the initialization.

Previously this was impossible:

```py
special_tokens = [AddedToken('<special>')]

GPT2Tokenizer.from_pretrained('gpt2', additional_special_tokens=special_tokens)
```

This enables this functionality and adds a test. This also fixes all tokenizers that were ill-configured for that purpose..
@sgugger, please merge if you're happy with the updated PR :)