Enable putting data, assets, and layouts together

[willfaught]

Hugo should let users group content/, layouts/, and static/ files under content/.

It's much simpler for beginners, when starting fresh, and when porting an existing static site incrementally.

E.g.

mysite/
  content/
    _baseof.html
    _list.html
    _single.html
    about.md
    me.jpg
    style.css
    blog/ # inherits _baseof.html from content/
      _list.html
      _single.html
      foo.md
      articles/ # inherits _baseof.html from content/, and _list.html from blog/
        _single.html
        bar.md

Then layouts/ can be for when you want to factor out patterns into common templates:

mysite/
  layouts/
    _baseof.html
    _list.html
    _single.html
    blog/
      _single.html
      _list.html