Feature: support converting between inline links and reference links

[gibfahn]

See https://stackoverflow.com/questions/30917399/convert-markdown-links-from-inline-to-reference for a full example.

Basically looking for an automated way to go from:

[text](url)

... rest of doc

To:

[text][]

... rest of doc

[text]: url

[gibfahn]

There are lots of more complicated things that could be done, like not creating the [text]: url link definition if it already exists, or using a custom name like:

[text][label]

... rest of doc

[label]: url

but this simple version would cover the common use-case.