tpl/collections: Add collections.Append

Before this commit you would typically use `.Scratch.Add` to manually create slices in a loop.

With variable overwrite in Go 1.11, we can do better. This commit adds the `append` template func.

A made-up example:

```bash
{{ $p1 := index .Site.RegularPages 0 }}{{ $p2 := index .Site.RegularPages 1 }}
{{ $pages := slice }}
{{ if true }}
  {{ $pages = $pages | append $p2 $p1 }}
{{ end }}
```

Note that with 2 slices as arguments, the two examples below will give the same result:

```bash
{{ $s1 := slice "a" "b" | append (slice "c" "d") }}
{{ $s2 := slice "a" "b" | append "c" "d" }}
```

Both of the above will give `[]string{a, b, c, d}`.

This commit also improves the type handling in the `slice` template function. Now `slice "a" "b"` will give a `[]string` slice. The old behaviour was to return a `[]interface{}`.

Fixes #5190

common/collections/collections.go

@@ -24,5 +24,5 @@ type Grouper interface {
 // in collections.Slice template func to get types such as Pages, PageGroups etc.
 // instead of the less useful []interface{}.
 type Slicer interface {
-	Slice(items []interface{}) (interface{}, error)
+	Slice(items interface{}) (interface{}, error)
 }