Support "nosig" shortcode to hide signature in post

If a post contains <!--nosig--> anywhere in the body, the post will render
without a post signature on it.

Ref T815

postrender.go

@@ -64,6 +64,10 @@ func (p *Post) augmentContent(c *Collection) {
 		// Don't augment posts that are pinned
 		return
 	}
+	if strings.Index(p.Content, "<!--nosig-->") > -1 {
+		// Don't augment posts with the special "nosig" shortcode
+		return
+	}
 	// Add post signatures
 	if c.Signature != "" {
 		p.Content += "\n\n" + c.Signature