Add allowed-special

[FergusFettes]

I got an error cause a file I was trying to count the tokens of contained some special tokens. This is a quick fix for that. Happy to make some modifications or write a test if you are interested in this change.

Before:

echo '<|endoftext|>' | ttok
> Long tiktoken error

Now:

echo '<|endoftext|>' | ttok --allowed-special='<|endoftext|>'
2

[simonw]

Thanks for the PR! I ended up solving this a slightly different way:

• Options for special token handling #13