Icons path option for overriding default icons path

I have added an iconsPath options. It overrides the 'filePath(name)' logic only if it is a string, not falsy.

My use case was the following: I use the module in a gulp task: the favicon source image is located in a source folder and generates the favicons in a dist folder (served). paths were wrong ('../../dist' ...) when generating the icons in a parent then siblings folder from the favicon source image.
@DamienSeguin Just realised after merging, you should be joining these e.g.

```
path.join(options.files.iconsPath, opts.name);
```

Otherwise we get things like `.//image.png`. Can you PR a fix?