Add Post Cover Title Option to posts #47

@raamdev I'm not a wordpress developer that often, so I'm not sure if I did this feature with the best approach. But I created the option to check if the post has the "independent_publisher_post_cover_title" and "independent_publisher_post_cover_subtitle" custom fields, as you said on the thread #47, and also added the option to "Auto-Set Post with Post Cover Title" option in the general options of the theme.

I also added the js script to make the title fade while scrolling the window, just like at Medium.

For the CSS I added the class "post-has-post-cover-title" to the body, so the user can customize when the Post Cover Title is enabled.

I also created the translation to Brazilian Portuguese, and updated the Default to include the string "'Auto-Set post with Post Cover Title style" that I'll create another pull request.

Again, I'm not sure if this is the best approach and if a did right, so if you could please provide me some feedbacks, I will be more than happy to update the code if needed.

Thanks,
Gabriel
This function description comment should be updated to reflect what the function does (it's currently using the `independent_publisher_full_width_featured_image_body_class()` description.)