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".

Start by locating the ion-icon component and how it handles child markup and styling. Assess the requested indicator slot using the example in the issue; done means an indicator icon can render inside an ion-icon without requiring a custom component or breaking its use in items and buttons.

Written by the indexing model from the issue text.