plainText (el) {
    var state = this.state
    var text = el.textContent
    if (state.stack.length > 0) {
      var context = last(state.stack)
      context.offset += text.length
      context.text += context.text.concat(text)
    }
    return text
  }
Converts the given element as plain-text.

@param {DOMElement} el
@returns {String} The plain text