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

output/outputFormat.go

@@ -33,6 +33,7 @@ var (
 		IsHTML:    true,
 	}
 
+	// CalendarFormat is AAA
 	CalendarFormat = Format{
 		Name:        "Calendar",
 		MediaType:   media.CalendarType,
@@ -104,6 +105,45 @@ func (formats Formats) GetByName(name string) (f Format, found bool) {
 	return
 }
 
+func (formats Formats) GetBySuffix(name string) (f Format, found bool) {
+	for _, ff := range formats {
+		if name == ff.MediaType.Suffix {
+			if found {
+				// ambiguous
+				found = false
+				return
+			}
+			f = ff
+			found = true
+		}
+	}
+	return
+}
+
+func (formats Formats) FromFilename(filename string) (f Format, found bool) {
+	// mytemplate.amp.html
+	// mytemplate.html
+	// mytemplate
+	var ext, outFormat string
+
+	parts := strings.Split(filename, ".")
+	if len(parts) > 2 {
+		outFormat = parts[1]
+		ext = parts[2]
+	} else if len(parts) > 1 {
+		ext = parts[1]
+	}
+
+	if outFormat != "" {
+		return formats.GetByName(outFormat)
+	}
+
+	if ext != "" {
+		return formats.GetBySuffix(ext)
+	}
+	return
+}
+
 // Format represents an output representation, usually to a file on disk.
 type Format struct {
 	// The Name is used as an identifier. Internal output formats (i.e. HTML and RSS)