Respect app mounts and improve view compilation

Thanks for building a cool blogging platform :+1: 

The changes make it possible to use poet as a mounted sub-app:

```js
mainApp.use("/private-blog", Poet(express(), {posts: './_private-posts'}));
mainApp.use("/public-blog", Poet(express(), {posts: './_public-posts'}));
```

and to use includes as well as view locals in jade.

Error reporting of failing post compilations is improved as those aren't simply
ignored in non-production environments but the posts content is the error.