add contentFor 

Inspired by ejs-layout, I update the code to support `contentFor` block with following syntax:

# inside the view

    <%- contentFor('leftNav') -%>
    <ul>
     ...
    </ul>

    <%- contentFor('someOtherSection') -%>
    ....
# inside the layout

    <%- contentFor.leftNav -%>
    .. // snipped ...
    <%- contentFor.someOtherSection -%>