Qualify slugs with those from parent headings

In the example below, the ``<h2>`` element would get an id ``"hello-one-two-three"``.

    <div><h1>Hello</h1><h2>One Two &amp; Three</h2>

This solves a problem where headings with the same text would get the same id (which is invalid and breaks navigation).

![michael-jackson-eating-popcorn-o](https://cloud.githubusercontent.com/assets/26405/3286991/5c061d90-f551-11e3-9acc-9ff70696dc4a.gif)

This closes https://github.com/rstacruz/flatdoc/issues/32.