Added filter support

Add the possibility to run the input through series of transformation filters (such as markdown) before it is included.

So, for example, you could run

    @@include(markdown('article.md'))

to include a markdown document and output it rendered.

You would register your filters by including them as an object in your `gulpfile`. For example

```
gulp.task('fileinclude', function() {
  gulp.src(['index.html'])
    .pipe(fileinclude({
      filters: {
        markdown: require("markdown").parse
      }
    }))
    .pipe(gulp.dest('./'));
}); 
```