Wrong behavior of string.format

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.

But Fengari raises error instead of assuming zero:
```
   > print(("%.0f"):format(42.5))
   43
   > print(("%.f"):format(42.5))
   SyntaxError: [sprintf] unexpected placeholder
```