Support posting songs

So far only audio files are supported, but we could also add support for
embeds from SoundCloud, and possibly other services.

go/blog/blog.go

			err = linkTmpl.Execute(os.Stdout, post)
		case "image":
			err = imageTmpl.Execute(os.Stdout, post)
		case "song":
			err = songTmpl.Execute(os.Stdout, post)
		default:
			fmt.Fprintf(os.Stderr, "Error: no output for type '%s'\n", post.Type)
			os.Exit(1)
</article>
`))
var songTmpl = template.Must(template.New("song").
	Funcs(funcs).Parse(`
<article id="{{ .Id }}" class="song">
	{{- if .Title }}
	<header>
		<h1>{{ .Title }}</h1>
		{{- if .Date }}<time>{{ .Date }}</time>{{ end -}}
	</header>
	{{- end }}
	<audio src="{{ safe_url .URL }}" controls>
		Your browser can't play {{ .URL }}.
	</audio>
	{{- if .Content }}
	{{ markdown .Content }}
	{{- end -}}
</article>
`))
func exit(err error) {
	fmt.Fprintf(os.Stderr, "Error: %s\n", err)
	os.Exit(1)

go/blog/blog.yaml

- url: file:///home/lu/m/pictures/inspiration/empartridge-anything.jpg
  content: By [@empartridge](http://empartridge.tumblr.com).
  type: image
- url: file:///home/lu/m/musique/DRALMS/Shook/05 - Shook.mp3
  content: Shook, by DRALMS.
  type: song