Added "subtemplating" for lack of a better work

The following changes allow the following:

Giving this function
```javascript
formTag = function (template) {
  return '<form>\n'+template()+'\n</form>';
}
```

The following ejs template:

```javascript
<%= formTag(function () { %>
    <input type="submit" />
<% }) %>
```

generates:

```html
<form>
    <input type="submit" />
</form>
```