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

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](https://github.com/editor-js/paragraph/blob/master/src/index.js#L189) whereas this header plugin doesn't.

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