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

tpl/tplimpl/templateProvider.go

@@ -0,0 +1,59 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tplimpl
+
+import (
+	"github.com/spf13/hugo/deps"
+)
+
+type TemplateProvider struct{}
+
+var DefaultTemplateProvider *TemplateProvider
+
+// Update updates the Hugo Template System in the provided Deps.
+// with all the additional features, templates & functions
+func (*TemplateProvider) Update(deps *deps.Deps) error {
+
+	newTmpl := newTemplateAdapter(deps)
+	deps.Tmpl = newTmpl
+
+	newTmpl.initFuncs()
+	newTmpl.loadEmbedded()
+
+	if deps.WithTemplate != nil {
+		err := deps.WithTemplate(newTmpl)
+		if err != nil {
+			newTmpl.addError("init", err)
+		}
+
+	}
+
+	newTmpl.MarkReady()
+
+	return nil
+
+}
+
+// Clone clones.
+func (*TemplateProvider) Clone(d *deps.Deps) error {
+
+	t := d.Tmpl.(*templateHandler)
+	clone := t.clone(d)
+
+	d.Tmpl = clone
+
+	clone.MarkReady()
+
+	return nil
+}