add ‘contexts’ option to allow one template to have multiple targets

It would be good to be able to build multiple files from one template without using one target per destination file. This is especially useful if you want to render multiple languages based on a single template.

I propose the contexts (note the s) option to solve this.

```JavaScript
dusthtml: {
    multiple_targets: {
        src: "templates/index.dust",
        dest: "dist/",
        options: {
            contexts: {
                "index.html.de": {
                    isoCode: "de-DE",
                    title: "Meine Seite"
                    /*...*/
                },
                "index.html.en": {
                    isoCode: "en-US",
                    title: "My Page"
                    /*...*/
                }
            }
        }
    }
}
```