Auto-generate HTML ids for headers

[mnapoli]

It's pretty useful to be able to link to a sub-section of a page. To do this, we need to have HTML IDs on headers, like this:

<h2 id="introduction">Introduction</h2>

This is possible with Markdown Extra (supported by Couscous by default):

## Introduction {#introduction}

But that requires to use that notation everywhere.

What would be good is to automatically add the IDs for headers. To do this, we can probably reuse what I wrote here in Piwik's documentation: TitleIdPreprocessor. We could implement that as a new step in Couscous.

[wysow]

@mnapoli is that a duplicate of #39 no?

[mnapoli]

Wow I completely forgot I already opened it haha