add support for markdown footnotes

Signed-off-by: Naman Sood <mail@nsood.in>

post.go

@@ -46,6 +46,7 @@ func newPost(slug string) (*Post, error) {
 			extension.Linkify,
 			extension.Strikethrough,
 			extension.Typographer,
+			extension.Footnote,
 			meta.Meta,
 			highlighting.Highlighting,
 		),

styles/main.scss

@@ -322,6 +322,10 @@ article {
 		text-align: center;
 		text-decoration: none;
 	}
+
+	hr {
+		margin: 2*$ls $ls $ls;
+	}
 }
 
 footer {