layouts_dir setting does not get higher priority in search order for layouts.

Middleman 3.2.2

in my config.rb:
```
page "/news/*", :layout => :news`
set :layouts_dir, 'layouts'
```

in my source directory:

source/layouts/news.haml
source/news.haml

But /news/* are always rendered with the latter file,
I have to choose another file name to avoid this situation.

BTW,  :layouts_dir  and :partials_dir could not be found in document on website.