[Feature] poptip supports custom html node

[xiaoluoHe]

What problem does this feature solve?

poptip 有一些自定义样式需求，vrender 提供的配置不能完全满足。
考虑样式需求多样化，poptip 支持自定义 html 节点能更好的提升定制能力。

What does the proposed API look like?

poptip: {
  html: {
      dom: "xxx"
   }
}