Added option to target a parent element (And subsequently only apply to child elements)

Hi,

I've altered **anchor.js** so it allows you to define an element to apply to. This way, only child elements are affected.

I've used this in my Jekyll blog (Not yet pushed) to stop anchor.js from altering the site header and other little headers around my site, and so it only targets the main content of my blog posts/pages.

If you don't provide the second argument in `addAnchor()`, it just acts as normal and targets the full document.

Hope that's alright, I've never actually created a pull request before!

Great work on anchor.js, and I hope you accept my request :smile:

Regards.