tpl: Rework to handle both text and HTML templates

Before this commit, Hugo used `html/template` for all Go templates.

While this is a fine choice for HTML and maybe also RSS feeds, it is painful for plain text formats such as CSV, JSON etc.

This commit fixes that by using the `IsPlainText` attribute on the output format to decide what to use.

A couple of notes:

* The above requires a nonambiguous template name to type mapping. I.e. `/layouts/_default/list.json` will only work if there is only one JSON output format, `/layouts/_default/list.mytype.json` will always work.
* Ambiguous types will fall back to HTML.
* Partials inherits the text vs HTML identificator of the container template. This also means that plain text templates can only include plain text partials.
* Shortcode templates are, by definition, currently HTML templates only.

Fixes #3221

hugolib/menu_test.go

@@ -18,6 +18,8 @@ import (
 
 	"fmt"
 
+	"github.com/spf13/afero"
+
 	"github.com/stretchr/testify/require"
 )
 
@@ -42,7 +44,7 @@ title = "Section Menu"
 sectionPagesMenu = "sect"
 `
 
-	th, h := newTestSitesFromConfig(t, siteConfig,
+	th, h := newTestSitesFromConfig(t, afero.NewMemMapFs(), siteConfig,
 		"layouts/partials/menu.html", `{{- $p := .page -}}
 {{- $m := .menu -}}
 {{ range (index $p.Site.Menus $m) -}}