Pass all formats via --extern for libs

While we support the `crate_type` key in the manifest, we need to pass through
all crate types to the `--extern` flag.

Closes #177
auto merge of #189 : alexcrichton/cargo/all-crate-types, r=wycats

While we support the `crate_type` key in the manifest, we need to pass through
all crate types to the `--extern` flag.

Closes #177