Add a --target option

[asomers]

When working on crates with OS-specific functionality, it's useful to cross-compile the docs. It would be even more useful if cargo-docserver allowed viewing the result. Something like this:

$ cargo doc --target=x86_64-unknown-linux-gnu
$ cargo docserver --target=x86_64-unknown-linux-gnu
Listening on http://0.0.0.0:4000
^C
$ cargo doc --target=x86_64-apple-darwin
$ cargo docserver --target=x86_64-apple-darwin