Add keys method to extractors

[craigpastro]

First attempt at #195. I could probably use some help with the documentation though 🙇

Closes #195.

[jtescher]

@siyopao great start! What do you think about having keys return an iterator over &strs instead of a Vec? Could help people avoid the additional allocation.

[craigpastro]

Thanks for the comment @jtescher. Good idea! I've updated to return an iterator.

[jtescher]

@siyopao Sorry I hadn't looked into this closely enough, I don't think we can have this be an iterator without additional allocations as the lifetimes for something like &dyn Iterator<Item=&str> can't be easily resolved. @TommyCpp any clever ideas here? If not then the original Vec<&str> is probably the better return type here.

[TommyCpp]

@siyopao Sorry I hadn't looked into this closely enough, I don't think we can have this be an iterator without additional allocations as the lifetimes for something like &dyn Iterator<Item=&str> can't be easily resolved.

I agree. There is no guarantee how extractors will store those keys. So we may as well use Vec

[craigpastro]

No worries. Reverted that commit.

[jtescher]

Ok looks good 👍