Support hiding the featured image header text. (#155)

I updated the documentaiton as well as changed the contact page in the
exampleSite to use this feature as a demo.

README.md

 
 For any page or post you can add a featured image by including the local path in front matter (see content in the `exampleSite/content/_readme.md` file for examples): `featured_image: '/images/gohugo-default-sample-hero-image.jpg'`
 
+If you would like to hide the header text on the featured image on a page, set `omit_header_text` to `true`. See `exampleSite/content/contact.md` for an example.
+
 You don't need an image though. The default background color is black, but you can change the color, by changing the default color class in the config.toml file. Choose a background color from any on the [Tachyons](http://tachyons.io/docs/themes/skins/) library site, and preface it with "bg-"
 
 example: `background_color_class = "bg-blue"` or `background_color_class = "bg-gray"`

exampleSite/content/contact.md

 ---
 title: Contact
+featured_image: "images/notebook.jpg"
+omit_header_text: true
 description: We'd love to hear from you
 type: page
 menu:

layouts/partials/page-header.html

     <div class="pb3-m pb6-l bg-black-60">
       {{ partial "site-navigation.html" . }}
       <div class="tc-l pv6 ph3 ph4-ns">
-        <h1 class="f2 f1-l fw2 white-90 mb0 lh-title">{{ .Title | default .Site.Title }}</h1>
-        {{ with .Params.description  }}
-          <h2 class="fw1 f5 f3-l white-80 measure-wide-l center lh-copy mt3 mb4">
-            {{ . }}
-          </h2>
+        {{ if not .Params.omit_header_text }}
+          <h1 class="f2 f1-l fw2 white-90 mb0 lh-title">{{ .Title | default .Site.Title }}</h1>
+          {{ with .Params.description  }}
+            <h2 class="fw1 f5 f3-l white-80 measure-wide-l center lh-copy mt3 mb4">
+              {{ . }}
+            </h2>
+          {{ end }}
         {{ end }}
       </div>
     </div>