Make the vinyl file available as template variable.

The vinyl file is now available as `_file` variable. The `_target` variable has two properties, the `_target.path` contains the full target path and `_target.relative` contains the relative target path.

I find this really useful, when piping multiple templates through gulp like this:
```javascript
gulp.task('build-pages', function () {
    return gulp
        .src(['pages/**/*.twig'], {base: 'pages'})
        .pipe(twig(...))
        ...;
});
```