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 
Or, maybe, fill it with: `#[phase(plugin)] extern crate spellck;`