Use id attribute instead of name for ToC

The `name` attribute is no longer considered a global attribute or part of the `a` element as of the XHTML/HTML5 spec. `id` has replaced this deprecated usage of the `name` attribute. `name` is now reserved only for "form controls".

> Note that in XHTML 1.0, the name attribute of these elements is formally deprecated, and will be removed in a subsequent version of XHTML. - http://www.w3.org/TR/xhtml1/#h-4.10

Related

* #64 cc @jakedouglas @brianmario
* The `a` element - http://www.w3.org/TR/html5/text-level-semantics.html#the-a-element
* The `id` attribute - http://www.w3.org/TR/html5/dom.html#the-id-attribute
* The `name` attribute - http://www.w3.org/TR/html5/forms.html#naming-form-controls:-the-name-attribute

cc @jch @raganwald @mislav