add sectioned content to document pages

Instead of using `content()`, you can get sectioned content using
`sections()`. This doesn't add anything to the default theme yet.

I'm not sure I like the syntax here. I considered doing `content(
sections => 2 );` or something, but this was easier.

Fixes #26
add content sections to the default templates

Now only the first section will appear on index/list pages and in the
feeds, and a bunch of links to continue reading will be shown.

Fixes #26