Allow HTML5 required attribute

[dsimandl]

It seems that HTML5lib is stripping out the required attribute when I try to write HTML5 in Django-CMS. I assume this is because of security concerns inside the html5lib library? The required attribute removes the need for JS validation and I consider it an important attribute in the HTML5 spec. I would be willing to look into what needs to change inside html5lib-python itself if necessary.

[willkg]

@dsimandl Can you provide example code that illustrates the problem?

[willkg]

If I can get either a failing test or an example to use, that'd be great. A PR would be great, too.

Until then, I'm taking this out of the 1.0 milestone.

[gsnedders]

(I presume this is the sanitizer stripping it)