Top-level `processIncludeContents` option

Because I need to apply a uniform transformation across both includes and "includers", I locally altered include-replace to optionally apply the `processIncludeContents` function not only to includes, but also to top-level files.

If this is welcome, I'd like to provide a patch for this.

``` js
includereplace: {
dist: {
options: {
processIncludeContents: function (src, locals) {
// do sthg!
},
// if this is thruthy, all levels will be processed with `processIncludeContents`
processTopLevel: true
},
src: '*.html',
dest: 'dist/'
}
}
```