Add a cargo-doc command

This is blocked until https://github.com/rust-lang/rust/pull/15939 lands, but in the meantime I figured I could get some eyes on this to make sure I'm sane.

Closes #130
The problem here is specifically the `rmdir`, since it is silently killing the whole directory; and it is very reasonable and fairly common to have a `./doc` directory with handwritten docs. e.g.

- https://github.com/basho/riak/tree/develop/doc
- https://github.com/clutchski/coffeelint/tree/master/doc
- https://github.com/scottschiller/SoundManager2/tree/master/doc
- https://github.com/jemalloc/jemalloc/tree/dev/doc

And there's a pile of projects using `./docs` too. About half of the projects I randomly clicked on in Github's ["showcases"](https://github.com/showcases) had either `./doc` or `./docs` as (AFAICT) human written docs.

I think it's a *really* bad idea to force-remove the directory, destroying people's work, and it seems suboptimal to put machine generated output into it by default (although I agree that `target` is a little weird for documentation).