Access data files from source folder

[gemedet]

This allows pages in the source folder to access properties in the source/_data files.

For example, in source/index.html:

<p>Hello {{site.data.foo.name}}</p>

[Xuanwo]

My be we can support two different data, one from site and the other from theme.
What do you think about that?
So I can access them like <p>Hello {{theme.data.foo.name}}</p>

[gemedet]

Sure, that'd give us more flexibility. I think it would be a separate PR, since you'd want to access the data from the theme's templates as well.