go.tools/present: add support for figure captions

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/110600043

cmd/present/static/styles.css

@@ -453,4 +453,9 @@ div.output .buttons {
 }
 iframe {
   border: none;
+}
+figcaption {
+  color: #666;
+  text-align: center;
+  font-size: 0.75em;
 }

cmd/present/templates/action.tmpl

@@ -44,3 +44,5 @@ It determines how the formatting actions are rendered.
 {{define "link"}}<p class="link"><a href="{{.URL}}" target="_blank">{{style .Label}}</a></p>{{end}}
 
 {{define "html"}}{{.HTML}}{{end}}
+
+{{define "caption"}}<figcaption>{{style .Text}}</figcaption>{{end}}

present/caption.go

@@ -0,0 +1,22 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package present
+
+import "strings"
+
+func init() {
+	Register("caption", parseCaption)
+}
+
+type Caption struct {
+	Text string
+}
+
+func (c Caption) TemplateName() string { return "caption" }
+
+func parseCaption(_ *Context, _ string, _ int, text string) (Elem, error) {
+	text = strings.TrimSpace(strings.TrimPrefix(text, ".caption"))
+	return Caption{text}, nil
+}

present/doc.go

@@ -61,6 +61,7 @@ After that come slides/sections, each after a blank line:
 	.iframe http://foo
 	.link http://foo label
 	.html file.html
+	.caption _Gopher_ by [[http://www.reneefrench.com][Renée French]]
 
 	Again, more text
 
@@ -176,6 +177,16 @@ preserves the aspect ratio of the image when scaling.
 
 	.image images/janet.jpg _ 300
 
+
+caption:
+
+The template uses the function "caption" to inject figure captions.
+
+The text after ".caption" is embedded in a figcaption element after
+processing styling and links as in standard text lines.
+
+	.caption _Gopher_ by [[http://www.reneefrench.com][Renée French]]
+
 iframe:
 
 The function "iframe" injects iframes (pages inside pages).