Export URL handler to allow for easier overriding.

[aprotim]

Description

Alternative to #1361, allowing users to replace Renderer's url cleaner themselves.

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[UziTech]

Why can't you just use renderer.link and renderer.image to do the same thing?

[styfle]

I agree with @UziTech

I think this is going to expose one more API that we will later have to remove when sanitization gets removed. I don't think this is the direction we want to go for the same reason as my comment in 1361.