Self-closing elements should be equivalent

Currently optionally self-closing elements such as images and line breaks are not considered equivalent. E.G.

``` js
const differ = require('html-differ')

let lhs = '

'
let rhs = ''

let diff = differ.diffHtml(lhs, rhs)

console.log(diff)

// [
// {
// "value": "

"
// }
// ]
```

For reference Parse5 and htmlparser2 interpret both strings equally. I'm looking into this to see if I can help.