Feature request: format using footnote-style links

[tgross35]

It seems like currently formatting always turns links into the [name](http://full.url) format. Would it be possible to add an option that instead does [name]: http://full.url?

If possible, the option to preserve the original style would be helpful.

[kivikakk]

I'd be happy for this feature to be implemented and would welcome PR(s) to do so, though thinking through it, there are many complicated edge-cases. I'd want to discuss the implementation plan/design before someone proceeded, as otherwise the risk of running into difficulty is high.