Allowing data to be set from a callback per file.

This is an alternate solution to #2.

This allows you to define a callback for the `data` option which will be called per file.

```js
function getTemplateDataForFile (file) {
    return {
        some_property : file.propertySetByAnotherTask,
        STATIC_URL : path.relative(file.base, pathToStaticAssets)
    };
}
gulp.src('*.html')
    .pipe(swig({
        data : getTemplateDataForFile
    }))
```