%.f fails with unexpected placeholder

https://github.com/fengari-lua/fengari/issues/147

C language docs says the following about `.precision` field in `sprintf()` format:

> If the period is specified without an explicit value for precision, 0 is assumed.

```
$ node
> require("sprintf-js").sprintf("%.f", 2)
Thrown:
SyntaxError: [sprintf] unexpected placeholder
> require("sprintf-js").sprintf("%.0f", 2)
'2'
```