Route in template

I have added facility to include actual routes in templates. Besides
being of independent interest, it can help in referring to resources
whose actual route changes depending as a function of compilation.
In particular, it is applicable in the context of issue #303.

With is pull request you can refer to actual routes in templates
as

```
$route("index.md")$ 
```

and actually get `index.html` in the output (provided the route of
`index.md` is `index.html`