va_list support
ImageMagick uses the type in these two functions:
```C
ThrowMagickExceptionList(ExceptionInfo *, const char *, const char *,
  const size_t, const ExceptionType, const char *, const char *, va_list),

LogMagickEventList(const LogEventType, const char *, const char *, const size_t,
  const char *, va_list);
```

First one in `exception.c` uses the value here (`operands` is the `va_list`):
`n = vsprintf(reason+length, format, operands);`

Currently nimterop just keeps the `va_list` type, but it's undeclared in Nim.

Maybe something like 
```nim
type va_list {.importc: "va_list", header:"<stdarg.h>".} = object
```