Feature request: Add indicator slot

Hi there.

It would be great if the ion-icon component would allow for an indicator slot so we could use ion-icon somehow like this:

```

```

This would allow us to assign markup to the inside of an ion-icon. I am currently trying to find a good solution to add icons with indicators (other icons on top of the existing one). And I can not really find a good solution without creating a custom component, which I do not want to because it breaks all my styling in items, buttons etc. that excpects an ion-icon component and not a "custom-component".

首先定位 ion-icon 组件，并了解它如何处理子标记和样式。使用 issue 中的示例评估所请求的 indicator slot；完成的标准是：indicator icon 可以在 ion-icon 内部渲染，无需自定义组件，也不会破坏其在 items 和 buttons 中的使用。

由索引模型根据 Issue 内容生成。