Add sub-parameters

Example

Gruntfile.js:

```javascript
    grunt.initConfig({
        
        ...

        htmlbuild: {
            dist: {
                src: 'index.html',
                ...
                options: {
                    ...
                    sections: {
                        products: {
                            vegetables: {
                                potatoes: 'potatoes.html',
                                tomatoes: 'tomatoes.html',
                            },

                            fruits: {
                                bananas: 'bananas.html',
                                apples: 'apples.html',
                                oranges: 'oranges.html'
                            }
                        },
                    },
                    ...
                }
            }
        }
        ...
    });
```
index.html:

```html
<h2>Vegetables:</h2>
<div class="vegetables">
	<!-- build:section products.vegetables -->
    <!-- /build -->
</div>

<h2>Fruits:</h2>
<div class="fruits">
	<!-- build:section products.fruits -->
    <!-- /build -->
</div>

<h2>All products:</h2>
<div class="products">
	<!-- build:section products -->
    <!-- /build -->
</div>
```