added optional method chain indentations for emacs major mode

I added an option to auto-indent method chains to line up along their '.' operators. Like so: 

``` 
let input = io::stdin().readline()
                       .ok()
                       .expect("Failed to read line");
```

The old default would indent like so:
```
let input = io::stdin().readme()
    .ok()
    .expect("Failed to read line");
```

The Rust guide explicitly condones the former, so I thought it would be nice for the emacs mode to support it. It's off by default, you have to set ```rust-indent-method-chain``` to ```t``` via your .emacs or the customize menu