Fix over zealous whitespace removal

This changes setHTML to use new method Normalizer.removeBlockWhitespace. That method removes whitespace between any block level opening or closing tag and another tag. This also moves 2 pieces of code formerly inside Normalizer.stripWhitespace so that they can be reused (by Document.setHTML): replaceNewLines and strip (which removes leading and trailing whitespace as pythons str.strip method does).

Addresses https://github.com/quilljs/quill/issues/149