Render unpublished posts (Fix #1675)

Fix #1675.

Based on the [documentation](http://octopress.org/docs/blogging/), we should be able to preview posts having `published: false` in its YAML header.
What do you think about turning `all` into an `*options` hash, such that I can call:

```ruby
preview_site(source_dir, "--unpublished")
```

and the method would just

```ruby
(%w{jekyll build --watch} + options).join(" ")
```