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/page.go

@@ -1384,13 +1384,14 @@ func (p *Page) prepareLayouts() error {
 	if p.Kind == KindPage {
 		if !p.IsRenderable() {
 			self := "__" + p.UniqueID()
-			_, err := p.s.Tmpl.GetClone().New(self).Parse(string(p.Content))
+			err := p.s.Tmpl.AddLateTemplate(self, string(p.Content))
 			if err != nil {
 				return err
 			}
 			p.selfLayout = self
 		}
 	}
+
 	return nil
 }