Generate ids for headings in markdown

[h-lame]

I am pretty sure that markdown generators can be asked to create id attributes based on the heading values when generating HTML. For example:

# Hats

all about hats

## Caps

caps are great

## Fedoras

these are bad

## Other Full Brimmed Hats

these are probably ok

would turn into:

<h1 id="hats">Hats</h1>
<p>all about hats</p>
<h2 id="caps">Caps</h2>
<p>caps are great</p>
<h2 id="fedoras">Fedoras</h2>
<p>these are bad</p>
<h2 id="other-full-brimmed-hats">Other Full Brimmed Hats</h2>
<p>these are probably ok</p>

This would let us link directly to individual sections of a blog post (e.g. individual links in our round up posts).

I think it's an option for our markdown renderer and so something that should just be a config change, not a big engineering effort.