Consolidate the Param methods

Maps in Viper, Hugo's config backing store, is now properly lower-cased not just on top level, the current situation.

While this is mostly a good thing, as you don't need to know the original casing to look up a value, it will be breaking for people doing direct lookups in the ´Site.Params` map.

We will try to find a solution to this "breakage", but the recommended method to get params values is via the `.Param` methods.

This method is now implemented on `Node`, `Page` and `Site` and is case-insensitive:

* Use `.Param "someKey" ` if you want page param with fall back to site param if not found on page.
* Use `.Site.Param "someKey"` to get a site param

See #2590

hugolib/page.go

@@ -194,12 +194,13 @@ func (p *Page) IsPage() bool {
 // 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.
+// This method is also implemented on Node and SiteInfo.
 func (p *Page) Param(key interface{}) (interface{}, error) {
 	keyStr, err := cast.ToStringE(key)
 	if err != nil {
 		return nil, err
 	}
+	keyStr = strings.ToLower(keyStr)
 	if val, ok := p.Params[keyStr]; ok {
 		return val, nil
 	}