Spec should not start an HTML block for void elements

[glebm]

Void elements as described in the HTML spec should not start a CommonMark HTML block, because void elements cannot have any content.

The void elements are:

area base br col embed hr img input link menuitem meta param source track wbr

E.g. neither of these should start an HTML block, but currently do as they match HTML block Start condition 7:

<img src="x.jpg>

<img src="x.jpg" />

[glebm]

I propose the spec be changed to something like this: #436