Added a mach command for cargo update

As proposed in #3736

Example usage:

``` sh
$ ./mach update-cargo -p rust-xml
.
    Updating git repository `https://github.com/netvl/rust-xml`
ports/cef
    Updating git repository `https://github.com/netvl/rust-xml`
ports/android/glut_app
    Updating git repository `https://github.com/netvl/rust-xml`
```
auto merge of #4045 : mttr/servo/mach_update, r=jdm

Example usage:

``` sh
$ ./mach update-cargo -p rust-xml
.
    Updating git repository `https://github.com/netvl/rust-xml`
ports/cef
    Updating git repository `https://github.com/netvl/rust-xml`
ports/android/glut_app
    Updating git repository `https://github.com/netvl/rust-xml`
```