fix generated TOC

[trepmal]

resolves #4

This PR removes h4 tags from the default TOC generation, as they were not added correctly.

Now, h3 is used by default and is filterable, allowing h4 tags to be added like so:

add_filter( 'toc_tags_for_toc', function( $tags) {
	return [ 'h3', 'h4' ];
}, 11 );