Render the shortcodes as late as possible

This is needed to make shortcode users happy with the new multilanguage support,
but it will also solve many other related posts about "stuff not available in the shortcode".

We will have to revisit this re the handler chain at some point, but that will be easier
now as the integration test story has improved so much.

As part of this commit, the site-building tests in page_test.go is refreshed, they now
tests for all the rendering engines (when available), and all of them now uses the
same code-path as used in production.

Fixes #1229
Fixes #2323
Fixes ##1076

hugolib/shortcode.go

@@ -160,7 +160,11 @@ func HandleShortcodes(stringToParse string, page *Page, t tpl.Template) (string,
 	}
 
 	if len(tmpShortcodes) > 0 {
-		tmpContentWithTokensReplaced, err := replaceShortcodeTokens([]byte(tmpContent), shortcodePlaceholderPrefix, tmpShortcodes)
+		shortcodes, err := executeShortcodeFuncMap(tmpShortcodes)
+		if err != nil {
+			return "", err
+		}
+		tmpContentWithTokensReplaced, err := replaceShortcodeTokens([]byte(tmpContent), shortcodePlaceholderPrefix, shortcodes)
 
 		if err != nil {
 			return "", fmt.Errorf("Fail to replace short code tokens in %s:\n%s", page.BaseFileName(), err.Error())
@@ -274,7 +278,7 @@ func renderShortcode(sc shortcode, parent *ShortcodeWithPage, p *Page, t tpl.Tem
 	return renderShortcodeWithPage(tmpl, data)
 }
 
-func extractAndRenderShortcodes(stringToParse string, p *Page, t tpl.Template) (string, map[string]string, error) {
+func extractAndRenderShortcodes(stringToParse string, p *Page, t tpl.Template) (string, map[string]func() (string, error), error) {
 
 	if p.rendered {
 		panic("Illegal state: Page already marked as rendered, please reuse the shortcodes")
@@ -297,15 +301,32 @@ func extractAndRenderShortcodes(stringToParse string, p *Page, t tpl.Template) (
 
 }
 
-func renderShortcodes(shortcodes map[string]shortcode, p *Page, t tpl.Template) map[string]string {
-	renderedShortcodes := make(map[string]string)
+var emptyShortcodeFn = func() (string, error) { return "", nil }
+
+func executeShortcodeFuncMap(funcs map[string]func() (string, error)) (map[string]string, error) {
+	result := make(map[string]string)
+
+	for k, v := range funcs {
+		s, err := v()
+		if err != nil {
+			return nil, fmt.Errorf("Failed to execute shortcode with key %s: %s", k, err)
+		}
+		result[k] = s
+	}
+
+	return result, nil
+}
+
+func renderShortcodes(shortcodes map[string]shortcode, p *Page, t tpl.Template) map[string]func() (string, error) {
+	renderedShortcodes := make(map[string]func() (string, error))
 
 	for key, sc := range shortcodes {
 		if sc.err != nil {
 			// need to have something to replace with
-			renderedShortcodes[key] = ""
+			renderedShortcodes[key] = emptyShortcodeFn
 		} else {
-			renderedShortcodes[key] = renderShortcode(sc, nil, p, t)
+			shorctode := sc
+			renderedShortcodes[key] = func() (string, error) { return renderShortcode(shorctode, nil, p, t), nil }
 		}
 	}