The `title_format` configuration option assumes restructuredtext

[SmileyChris]

The correct solution to my issue in #607 is to use the title_format configuration option.

The only problem is that this option assumes that you want:

<formatted_title>
<config.underscores[0] * formatted_title_length>
<rendered content>

But for markdown, we only want:

# <formatted_title>
<rendered content>

The builder however doesn't have a concept of markdown or restructuredtext.

I think the most compatible solution would be to use the markdown method if the template ends with '.md', otherwise use the current method, correct?