[WIP] add filename replacement test

closed #42

This will work but not modify assemble context:
* `filename`
* `fileName`
* `basename`
* `page.data.date`

The reason is to use with other plugins/helpers/collections. Example scenario why we need to modify:
```
{{#each pages }}
<li{{#if this.isCurrentPage}} class="active"{{/if}}>
    <a href="{{{filename}} }">{{{title}} }</a>
</li>
{{/each }}
```
In this case `isCurrentPage` will never `true`, and `filename` (link) also always wrong.