Makes Paginate accept slices of PageGroup (in addition to PagesGroup)

User can now do something such as:
  {{ $cool := .Page.Group "cool" (where .Site.RegularPages "Params.cool" true) }}
  {{ $blue := .Page.Group "blue" (where .Site.RegularPages "Params.blue" true) }}
  {{ $paginator := .Page.Paginate (slice $cool $blue) }}
  {{ range $paginator.PageGroups }}
  {{   markdownify (printf "## KEY: %s" .Key) }}
  {{   range .Pages }}
  {{     partial "lists/partials/blogentry.html" (dict "Page" .) }}
  {{   end }}
  {{ end }}

hugolib/pagination.go

@@ -399,7 +399,11 @@ func paginatePages(td targetPathDescriptor, seq interface{}, pagerSize int) (pag
 
 	var paginator *paginator
 
-	if groups, ok := seq.(PagesGroup); ok {
+	groups, err := toPagesGroup(seq)
+	if err != nil {
+		return nil, err
+	}
+	if groups != nil {
 		paginator, _ = newPaginatorFromPageGroups(groups, pagerSize, urlFactory)
 	} else {
 		pages, err := toPages(seq)
@@ -414,6 +418,39 @@ func paginatePages(td targetPathDescriptor, seq interface{}, pagerSize int) (pag
 	return pagers, nil
 }
 
+func toPagesGroup(seq interface{}) (PagesGroup, error) {
+	if seq == nil {
+		return nil, nil
+	}
+
+	switch seq.(type) {
+	case PagesGroup:
+		return seq.(PagesGroup), nil
+	case []PageGroup:
+		return PagesGroup(seq.([]PageGroup)), nil
+	case []interface{}:
+		list := seq.([]interface{})
+		l := len(list)
+		if l == 0 {
+			break
+		}
+		switch list[0].(type) {
+		case PageGroup:
+			pagesGroup := make([]PageGroup, len(list))
+			for i, ipg := range list {
+				if pg, ok := ipg.(PageGroup); ok {
+					pagesGroup[i] = pg
+				} else {
+					return nil, fmt.Errorf("unsupported type in paginate from slice, got %T instead of PageGroup", ipg)
+				}
+			}
+			return PagesGroup(pagesGroup), nil
+		}
+	}
+
+	return nil, nil
+}
+
 func toPages(seq interface{}) (Pages, error) {
 	if seq == nil {
 		return Pages{}, nil