_customText (text) {
    var state = this.state
    if (state.stack.length > 0) {
      var context = last(state.stack)
      context.offset += text.length
      context.text += context.text.concat(text)
    }
    return text
  }
Tells the converter to insert custom text.

During conversion of annotatedText this is used to insert different
text than taken from the DOM. E.g., for inline nodes we insert an invisible
character instead of the inner content.

@private
@param {String}