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/templateFuncster.go

@@ -0,0 +1,86 @@
+// 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 (
+	"fmt"
+	"html/template"
+	"strings"
+
+	bp "github.com/spf13/hugo/bufferpool"
+
+	"image"
+
+	"github.com/spf13/hugo/deps"
+)
+
+// Some of the template funcs are'nt entirely stateless.
+type templateFuncster struct {
+	funcMap        template.FuncMap
+	cachedPartials partialCache
+	image          *imageHandler
+
+	// Make sure each funcster gets its own TemplateFinder to get
+	// proper text and HTML template separation.
+	Tmpl templateFuncsterTemplater
+
+	*deps.Deps
+}
+
+func newTemplateFuncster(deps *deps.Deps, t templateFuncsterTemplater) *templateFuncster {
+	return &templateFuncster{
+		Deps:           deps,
+		Tmpl:           t,
+		cachedPartials: partialCache{p: make(map[string]interface{})},
+		image:          &imageHandler{fs: deps.Fs, imageConfigCache: map[string]image.Config{}},
+	}
+}
+
+// Partial executes the named partial and returns either a string,
+// when called from text/template, for or a template.HTML.
+func (t *templateFuncster) partial(name string, contextList ...interface{}) (interface{}, error) {
+	if strings.HasPrefix("partials/", name) {
+		name = name[8:]
+	}
+	var context interface{}
+
+	if len(contextList) == 0 {
+		context = nil
+	} else {
+		context = contextList[0]
+	}
+
+	for _, n := range []string{"partials/" + name, "theme/partials/" + name} {
+		templ := t.Tmpl.Lookup(n)
+		if templ != nil {
+			b := bp.GetBuffer()
+			defer bp.PutBuffer(b)
+
+			if err := templ.Execute(b, context); err != nil {
+				return "", err
+			}
+
+			switch t.Tmpl.(type) {
+			case *htmlTemplates:
+				return template.HTML(b.String()), nil
+			case *textTemplates:
+				return b.String(), nil
+			default:
+				panic("Unknown type")
+			}
+		}
+	}
+
+	return "", fmt.Errorf("Partial %q not found", name)
+}