Support for breaking long URLs in URL reference syntax

[dbuenzli]

In my docsets I have hundreds of long links into the www that break my 80 source columns limit. While I may never get to break those in the future, I would really like to be able to do so for the next hundreds one.

More precisely to be able to turn:

{{:https://github.com/ocaml/odoc/issues/865}this issue}

into

{{:https://github.com/ocaml/\
   odoc/issues/865}this issue}