Add rule to enforce links as reference links

As default deactivated. If active, raise error here:

I'm a paragraph with [a link](http://link-to-somewhere)

…but passes this:

I'm a paragraph with [a link][link-to-somewhere]

[link-to-somewhere]: http://link-to-somewhere

[waldyrious]

It would also be nice if this was configurable by the length of the URL — i.e. non-reference links shorter than N characters would not trigger this rule, with the default max length being 0.

[leetrout]

CommonMark deviates from this and says it is optional but it is mentioned explicitly in the original Daring Fireball docs:

Then, anywhere in the document, you define your link label like this, on a line by itself:

[id]: http://example.com/ "Optional Title Here"

That is:

Square brackets containing the link identifier (optionally indented from the left margin using up to three spaces); followed by a colon; followed by one or more spaces (or tabs); followed by the URL for the link; optionally followed by a title attribute for the link, enclosed in double or single quotes, or enclosed in parentheses.