New "template-parts" folder & Jetpack Infinite scroll

New file template organization + Jetpack Infinite Scroll support by @grappler.
This PR is referring to leopuleo #637, now closed, and discussion #619.

 
This is incorrect. The right function would look like this:

```php
function _s_infinite_scroll_render() {
	while ( have_posts() ) {
		the_post();
		get_template_part( 'template-parts/content', get_post_format() );
	}
} // end function _s_infinite_scroll_render
```