cargo doc: Optionally generate foo/index.html with foo/ links, instead of foo.html

[SimonSapin]

If rust-lang/rust#19600 happens, it should be exposed by cargo doc. A command-line switch should be available so that cargo doc can default to generating longer links that work for file://… local viewing, but e.g. a Travis-CI script can generate nicer-looking links for HTTP(S) publication.

[alexcrichton]

I believe this is more of a rustdoc issue than a Cargo issue as Cargo isn't the one generating any of these links, so I think this would be better placed in rust-lang/rust instead of here.

[SimonSapin]

The rustdoc issue is rust-lang/rust#19600, as linked from the first message. Assuming that one gets fixed, this issue is about exposing it somehow to cargo doc.