"include" feature

[kylecordes]

First, thanks for markdown-styles. It is a very nice tool, stands out as a high quality choice even among the (at least) hundreds of static site generator tools. A feature request though:

Currently there appears to be no way to include one markdown file in another. That is a very common and frequently useful feature in static site generation. Issue #37 asks for something close to this, but it seemed worth having an issue dedicated to just this specific feature.