The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:
```
BenchmarkDefaultConfigProvider/Viper-16 252418 4546 ns/op 2720 B/op 30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16 450756 2651 ns/op 1008 B/op 6 allocs/op
```
Fixes#8633Fixes#8618Fixes#8630
Updates #8591Closes#6680Closes#5192
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
