spellck_extra_words attribute

This has the spellck lint recognize the attribute "spellck_extra_words" so that extra words can be added from the crate root. usage is like so:

```rust
//lib.rs
#![spellck_extra_words="these are not in word list but are now"]
```

problem: the attribute is marked unused by rustc. How can we have rustc recognize the attribute as used?

thank you for your time :)

closes #8 
This should go outside, and there should be an error message for a malformed, attribute:

if name.get() == "spellck_extra_words" {
    if let LitStr(ref raw_words, _) = lit.node {
        self.words.extend(...)
    } else {
        cx.sess().span_err(attr.span, "malformed `spellck_extra_words` attribute")
    }
}