Don't try and resolve paths for base64 encoded images and files.

I have a CSS file which contains a base64 encoded image inlined via Data URL. Connect-assets currently warns me with the following error:

```
Can't resolve image path: data:font/svg;charset=utf-8;base64,PD94
bWwgdmVyc2lvbj0iMS4wIiBzdGFuZGFsb25lPSJubyI/Pgo8IURPQ1
RZUEUgc3ZnIFBVQkxJQyAiLS8vVzNDLy9EVEQgU1ZHIDEuMS8v
**snip**
RU4iICJodHRwOi8vd3d3LnczLm9yZy9HcmFwaGljcy9TVkcvMS4xL
0RURC9zdmcxMS5kdGQiID4KPHN2ZyB4bWxucz0iaHR0cDovL3d3
dy53My5vcmcvMjAwMC9zdmciPgo8bWV0YWRhdGE+ClRoaX==
```

This pull request checks to see if the image is a Data URL and if it is, does not try and resolve the path.