Create a vocabulary from a set of `tokens`.

def create(cls, tokens:Tokens, max_vocab:int, min_freq:int) -> 'Vocab':
        "Create a vocabulary from a set of `tokens`."
        freq = Counter(p for o in tokens for p in o)
        itos = [o for o,c in freq.most_common(max_vocab) if c >= min_freq]
        for o in reversed(defaults.text_spec_tok):
            if o in itos: itos.remove(o)
            itos.insert(0, o)
        return cls(itos)