Header not allowing `<br>` when shift+enter pressed

[mrspence]

This plugin does not allow the user to press shift + Enter to create an inline break. This is supported in the paragraph plugin but not in this one yet! :-)

I believe the issue resides within the sanitize method. The Paragraph plugin has a "br" sanatize rule whereas this header plugin doesn't.

Proposed fix: extend plugin config to allow enabling of br tags in Header.

[rx2347]

This would be really useful, especially if you have margins for h tags to get the spacing to the next content block.