avoid parsing FrontMatter markdown files for now

Code style suggestion, instead of having the success case indented, you can handle error first, then the rest without indenting.

See https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Indent_Error_Flow.

I.e.,

```Go
err := yaml.Unmarshal([]byte(text), &map[string]interface{}{})
if err == nil {
    return text
}

... the rest
```