title: html/template: "layout" is undefined
body: <p>I try to use martini framework with layout template:</p>
  
  <pre><code>package main
  
  import (
      "github.com/go-martini/martini"
      "github.com/martini-contrib/render"
  )
  
  func main() {
      m := martini.Classic()
  
      m.Use(render.Renderer(render.Options{Directory: "./templates",
          Layout:     "layout",
          Extensions: []string{".tmpl"},
      }))
  
      m.Get("/", func(r render.Render) {
          r.HTML(200, "mainPage", map[string]interface{}{"Title": "some title", "H1": "some h1"})
      })
  
      m.Run()
  }
  </code></pre>
  
  <p>In the same folder as this <code>main.go</code>  file I got folder <code>templates</code> with <code>layout.tmpl</code> file:</p>
  
  <pre><code>&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;&lt;/html&gt;
  &lt;head&gt;&lt;/head&gt;
  &lt;body&gt;  
      &lt;div id='left'&gt;
          {{template "left" .}}
      &lt;/div&gt;
      &lt;div id='right'&gt;
          {{template "right" .}}
      &lt;/div&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p>and <code>mainPage.tmpl</code> file:</p>
  
  <pre><code>{{define "left"}}
    left content
  {{end}}
  
  {{define "right"}}
      right content
  {{end}}
  </code></pre>
  
  <p>When I open <code>http://localhost:3000/</code> in browser I see error:
  <code>html/template: "layout" is undefined</code></p>