Add ability to do interrupted chaining

[maael]

Basic Example

var body = html.add('div'),
    content = body.add('p')
console.log(content.build())

Loop Example

var table = html.add('table')
for (var i = 0; i < 3; i++) {
  table.add('tr')
}
console.log(table.build())

Current Error

TypeError: Cannot read property 'parentNode' of undefined
  at appendToLastAtLevel (html.js:69:33)

:exclamation: Requires new test :exclamation: