Feature request: Improved "partials" support #72
Description
I would like to propose better support for partials in nanoc. Our nanoc site has several complex pages that are broken into separate partials and then collated into a page. The current way that nanoc supports this is by defining the partials as "layouts" and then calling render on them. This works, but has the unfortunate effect that the pages are split across two directories; the partials in the layouts directory and the rest of the page in the content directory. Keeping them together would make logical sense and avoid navigation.
I found one workaround: You can define an additional data source and point the layout and content directory to the same place, and then hide all the partials from the content with a compilation rule (E.g. start each partial with an underscore, and ignore those). This works but feels messy and is hard to discover (we could document it I guess).
The most straightforward way to avoid the extra data source would be to make render look for all items, not just layouts. Looking at the source of render that seems fairly easy, but perhaps there is a technical reason I am overlooking that would make this hard to implement?