Custom event for render finish

[micahlt]

Is your feature request related to a problem? Please describe.
In my use case, I need to add target="_blank" attributes to all of the <a> elements in the <Markdown> component.

Describe the solution you'd like
It would be very helpful to fire a custom rendered event that would allow the parent component to know when the render is complete so that syntax could look like this:

<Markdown :source="source" @rendered="myFunction" />

Describe alternatives you've considered
Nothing as good as adding a custom event. I'm open to adding this feature in a pull request if that's an option.