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/pathspec_test.go

@@ -57,6 +57,6 @@ func TestNewPathSpecFromConfig(t *testing.T) {
 	require.Equal(t, "thethemes", p.themesDir)
 	require.Equal(t, "thelayouts", p.layoutDir)
 	require.Equal(t, "thework", p.workingDir)
-	require.Equal(t, "thestatic", p.staticDir)
+	require.Equal(t, "thestatic", p.StaticDir())
 	require.Equal(t, "thetheme", p.theme)
 }