Added a variable for excluding pages from build

With this you can use frontmatter to disable rendering of a page when needed. (Useful for prototyping)

```
---
title: Just testing some stuff on this page, it should not be published
published: false
---
```
I explained the justification for this in issue #122

The actual implementation may need some finesse and maybe the variable name should be changed to something else. (like ```publish: false```)