Remove play links for pkgs outside std lib

Fixes #308.
diff --git a/gddo-server/assets/templates/pkg.html b/gddo-server/assets/templates/pkg.html
index 3f9883d..9bd8f68 100644
--- a/gddo-server/assets/templates/pkg.html
+++ b/gddo-server/assets/templates/pkg.html
@@ -171,7 +171,7 @@
         <div class="panel-heading"><a class="accordion-toggle" data-toggle="collapse" href="#ex-{{.ID}}">Example{{with .Example.Name}} ({{.}}){{end}}</a></div>
         <div id="ex-{{.ID}}" class="panel-collapse collapse"><div class="panel-body">
           {{with .Example.Doc}}<p>{{.|comment}}{{end}}
-          <p>Code:{{if .Example.Play}}<span class="pull-right"><a href="?play={{.ID}}">play</a>&nbsp;</span>{{end}}
+          <p>Code:{{if .Play}}<span class="pull-right"><a href="?play={{.ID}}">play</a>&nbsp;</span>{{end}}
           {{code .Example.Code nil}}
           {{with .Example.Output}}<p>Output:<pre>{{.}}</pre>{{end}}
         </div></div>
diff --git a/gddo-server/template.go b/gddo-server/template.go
index 8a50a44..272dbf0 100644
--- a/gddo-server/template.go
+++ b/gddo-server/template.go
@@ -85,6 +85,7 @@
 	ID      string
 	Label   string
 	Example *doc.Example
+	Play    bool
 	obj     interface{}
 }
 
@@ -121,7 +122,14 @@
 		id += "-" + method
 	}
 	for _, e := range examples {
-		te := &texample{Label: label, ID: id, Example: e, obj: obj}
+		te := &texample{
+			Label:   label,
+			ID:      id,
+			Example: e,
+			obj:     obj,
+			// Only show play links for packages within the standard library.
+			Play: e.Play != "" && gosrc.IsGoRepoPath(pdoc.ImportPath),
+		}
 		if e.Name != "" {
 			te.Label += " (" + e.Name + ")"
 			if method == "" {