load Vocabulary from local path

### What behavior of the library made you think about the improvement?

I have a local tokenizer at, let's say, `/tmp/tokenizer1/`. I want to use it with `outlines-core`.

I tried `Vocabulary.from_pretrained('/tmp/tokenizer1/')`, but it said it couldn't find the tokenizer at huggingface.

### How would you like it to behave?

Support local tokenizer.