tpl/collections: Add group template func

This extends the page grouping in Hugo with a template function that allows for ad-hoc grouping.

A made-up example:

```
{{ $cool := where .Site.RegularPages "Params.cool" true | group "cool" }}
{{ $blue := where .Site.RegularPages "Params.blue" true | group "blue" }}
{{ $paginator := .Paginate (slice $cool $blue) }}
```

Closes #4865

common/collections/collections.go

@@ -0,0 +1,21 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package collections contains common Hugo functionality related to collection
+// handling.
+package collections
+
+// Grouper defines a very generic way to group items by a given key.
+type Grouper interface {
+	Group(key interface{}, items interface{}) (interface{}, error)
+}

common/types/types.go

@@ -1,4 +1,4 @@
-// Copyright 2017-present The Hugo Authors. All rights reserved.
+// Copyright 2018 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.