Renderer precedence, blocks, and template refs

[justinfagnani]

We need to figure out the semantics for template calls and precedence. Right now a block renders the fallback content unless it is overridden by a sub-template, but it should be able to defer to passed-in renderers as well. Currently they are aliased by the template's own blocks, so there's no way to do that.

We may need a <template call=> syntax to differentiate between block definitions and block calls, or some more complex lookup mechanism.

We should look at how Jinja handles this.