Add support for multiple staticDirs

This commit adds support for multiple statDirs both on the global and language level.

A simple `config.toml` example:

```bash
staticDir = ["static1", "static2"]
[languages]
[languages.no]
staticDir = ["staticDir_override", "static_no"]
baseURL = "https://example.no"
languageName = "Norsk"
weight = 1
title = "På norsk"

[languages.en]
staticDir2 = "static_en"
baseURL = "https://example.com"
languageName = "English"
weight = 2
title = "In English"
```

In the above, with no theme used:

the English site will get its static files as a union of "static1", "static2" and "static_en". On file duplicates, the right-most version will win.
the Norwegian site will get its static files as a union of "staticDir_override" and "static_no".

This commit also concludes the Multihost support in #4027.

Fixes #36
Closes #4027

hugolib/hugo_sites_build_test.go

@@ -1035,7 +1035,7 @@ func createMultiTestSitesForConfig(t *testing.T, siteConfig testSiteConfig, conf
 
 	if err := afero.WriteFile(mf,
 		filepath.Join("layouts", "_default/list.html"),
-		[]byte("{{ $p := .Paginator }}List Page {{ $p.PageNumber }}: {{ .Title }}|{{ i18n \"hello\" }}|{{ .Permalink }}"),
+		[]byte("{{ $p := .Paginator }}List Page {{ $p.PageNumber }}: {{ .Title }}|{{ i18n \"hello\" }}|{{ .Permalink }}|Pager: {{ template \"_internal/pagination.html\" . }}"),
 		0755); err != nil {
 		t.Fatalf("Failed to write layout file: %s", err)
 	}