Support specifying a rustc channel via +channel argument

`cargo` supports specifying channels via:
```
cargo +nightly bench
```

At the moment `cross` just runs cargo on the host system. As a workaround it is possible to specify channel via an environment variable:
```
RUSTUP_TOOLCHAIN=nightly cross bench
```

Think it would be nice to have this feature in order to be more `cargo` friendly.