Extend `#[deriving]` to allow options to be specified

This would make `deriving` significantly more flexible, e.g. for #3740:

``` rust
#[deriving(Encodable(encoder="RustcEncoder"))]
struct Foo { ... }
```

And for specifying an order of fields to sort by:

``` rust
#[deriving(Ord(bar, baz))]
struct Foo { baz: uint, bar: uint }
```
