Adds post number and total posts to posts listing

### Code

This adds post number and total posts to the `posts.php` page. This helped me do a bit more customization to my posts listing, and was a really simple addition.

### Reasoning
 
In my template for my post listing, I wanted to be able to know the current post number and the total posts. This was so I could build some HTML around the listing to make it display in columns.

This was a super simple addition to the `index.php` file that someone else might find useful, so I thought I'd post the pull request.