Only allow h3-h6 when saving posts. Tweak editor to only have h3-h6

See: https://github.com/GigaOM/gigaom/issues/5430 and https://github.com/GigaOM/gigaom/issues/5359
Sounds good. I've updated it to be aware of what has been set already.  I tested the updated code like so:

```php
add_filter( 'tiny_mce_before_init', function( $init ) {
  $init['block_formats'] = 'Paragraph=p;Address=addressPre=pre;Heading 1=h1;Heading 2=h2;Heading 3=h3;Heading 4=h4;Heading 5=h5;Heading 6=h6';

  return $init;
}, 9 );
```

The end result added `address` back in but kept `h1` and `h2` out as desired.