Closed #4: Adds layout option and {{yield}} template extensions support....

... Version bump to 0.0.4.

To use `{{yield}}`, add a layout option to your `config.json` like so:

    "handlebars": {
        "layout": "layout.html"
    }

Then make a `layout.html` file with contents like the following:

    <html>
    <head>
        <title>{{title}}</title>
    </head>
    <body>
        {{yield}}
    </body>
    </html>

Then when wintersmith renders your template, it will render `layout.html` first and fill `{{yield}}` with your template's data for each page.

This is an alternative to the classes mustache way of doing layouts which is kinda crummy:

    {{> header.html}}
    my content
    {{> footer.html}}