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

helpers/path_test.go

@@ -59,7 +59,8 @@ func TestMakePath(t *testing.T) {
 		v := viper.New()
 		l := NewDefaultLanguage(v)
 		v.Set("removePathAccents", test.removeAccents)
-		p, _ := NewPathSpec(hugofs.NewMem(v), l)
+		p, err := NewPathSpec(hugofs.NewMem(v), l)
+		require.NoError(t, err)
 
 		output := p.MakePath(test.input)
 		if output != test.expected {