Add Lastmod field

Create new field in Node
Update Page to look for lastmod field in the front matter. If not present, then assign Date to Lastmod
Update Site, to assign a value to Lastmod (based on the same logic used for Date)

Fixes #733

hugolib/page.go

@@ -473,6 +473,11 @@ func (p *Page) update(f interface{}) error {
 			if err != nil {
 				jww.ERROR.Printf("Failed to parse date '%v' in page %s", v, p.File.Path())
 			}
+		case "lastmod":
+			p.Lastmod, err = cast.ToTimeE(v)
+			if err != nil {
+				jww.ERROR.Printf("Failed to parse lastmod '%v' in page %s", v, p.File.Path())
+			}
 		case "publishdate", "pubdate":
 			p.PublishDate, err = cast.ToTimeE(v)
 			if err != nil {
@@ -524,6 +529,11 @@ func (p *Page) update(f interface{}) error {
 			}
 		}
 	}
+
+	if p.Lastmod.IsZero() {
+		p.Lastmod = p.Date
+	}
+
 	return nil
 
 }