Allow declaration of default block content

Using the usual handlebars block syntax:

    {{#block 'my-block'}}
      default content
    {{/block}}

"default content" will be shown at the location of the block declaration if no `{{#contentFor 'my-block'}}` was provided.

It is still possible to have an empty block, just declare:

    {{#contentFor 'my-block'}}
    {{/contentFor}}