c2nim fails on -1 in enums

The following code is not translated by c2nim:

``` c
enum
{
/* 8bit, color or not */
    CV_LOAD_IMAGE_UNCHANGED  =-1,
/* 8bit, gray */
    CV_LOAD_IMAGE_GRAYSCALE  =0,
/* ?, color */
    CV_LOAD_IMAGE_COLOR      =1,
/* any depth, ? */
    CV_LOAD_IMAGE_ANYDEPTH   =2,
/* ?, any color */
    CV_LOAD_IMAGE_ANYCOLOR   =4
};
```
