Allow all type of object in Typesetter#[] method.

Dear contributors of artii,

I modified behavior of Typesetter#[] method.
Because I want to use artii like following code.

```ruby
a = Artii::Base.new(font: "big")
a.asciify(123)
```
That execution will fail without my modifications because `123` is not a String object.
Please merge this modification if it is acceptable.

Regards,
Moza USANE