Option to customize fallback behavior if provider not available

[danjac]

At present if no provider is found for a URL, and urlize_all is True, the urlize function appears to always be called which renders a simple link. There doesn't appear to be a way to change this.

I'd like to be able to customize this fallback behavior - perhaps by passing in a function as is done with the handlers - for example if I want to render the link with target="_blank", or use the domain instead of the full URL in the title, etc.

Is there a way to do this at present?

[coleifer]

I've somewhat addressed this by allowing an additional parameter to the relevant methods that consists of arbitrary key-value pairs that get turned into attribute pairs on the resultant tag.

Lots of other changes, too

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