numeration start of images

option to set the starting point of the image numberation if your numberation doesn't start with 1 and you are to lazy to rename all your files ;-).

for example if your images look like this:
IMG_0145.jpg
IMG_0146.jpg
IMG_0147.jpg
...
```javascript
ThreeSixty({
  filePrefix: 'IMG_',
  numerationStart: 145
});
```