Adhering to HTML spec: optional tags

[MrFaul]

This is a reopening of #243

Well HTMLHint isn't able to deal with optional tags, aka tags that can be omitted.
It isn't alone in that i.e. VScode f's up its Breadcrumps completely. (It's on their todolist)

The spec is pretty clear on omissions, so I think HTMLHint should respect that.

It would be really nice if tables would stop yelling at me 😉