Add Param(key) to Node and Page

This  is a convenience method to do lookups in Page's (Page only)  and Site's Params map (Page and Node), in that order.

Fixes gohugoio#1462

hugolib/node.go

@@ -14,6 +14,7 @@
 package hugolib
 
 import (
+	"github.com/spf13/cast"
 	"html/template"
 	"sync"
 	"time"
@@ -86,6 +87,17 @@ func (n *Node) IsMenuCurrent(menuID string, inme *MenuEntry) bool {
 	return false
 }
 
+// Param is a convenience method to do lookups in Site's Params map.
+//
+// This method is also implemented on Page.
+func (n *Node) Param(key interface{}) (interface{}, error) {
+	keyStr, err := cast.ToStringE(key)
+	if err != nil {
+		return nil, err
+	}
+	return n.Site.Params[keyStr], err
+}
+
 func (n *Node) Hugo() *HugoInfo {
 	return hugoInfo
 }

hugolib/page.go

@@ -132,6 +132,21 @@ func (p *Page) IsPage() bool {
 	return true
 }
 
+// Param is a convenience method to do lookups in Page's and Site's Params map,
+// in that order.
+//
+// This method is also implemented on Node.
+func (p *Page) Param(key interface{}) (interface{}, error) {
+	keyStr, err := cast.ToStringE(key)
+	if err != nil {
+		return nil, err
+	}
+	if val, ok := p.Params[keyStr]; ok {
+		return val, nil
+	}
+	return p.Site.Params[keyStr], nil
+}
+
 func (p *Page) Author() Author {
 	authors := p.Authors()