start adding feature specs for blog post preview

~~WIP~~ This is for  #246
also closes: #259 #253 #252 #249
You can also do

```
add_column :posts, :draft, :boolean, null: false, default: false
```

Then all existing rows will be set to the default, false. Allowing NULL for a simple boolean flag like this also doesn't really make sense, I always try to disallow NULL in cases like this. NULL wouldn't have any specific meaning.