Add get_opt to std::vec

This adds `get_opt` to `std::vec`, which looks up an item by index and returns an `Option`. If the given index is out of range, `None` will be returned, otherwise a `Some`-wrapped item will be returned.

Example use case:

```rust
use std::os;

fn say_hello(name: &str) {
  println(fmt!("Hello, %s", name));
}

fn main(){
  // Try to get the first cmd line arg, but default to "World"
  let args = os::args();
  let default = ~"World";
  say_hello(*args.get_opt(1).unwrap_or(&default));
}
```

If there's an existing way of implementing this pattern that's cleaner, I'll happily close this. I'm also open to naming suggestions (`index_opt`?)