Facilitate themes to configure partials

[westonruter]

Instead of a big switch statement with hard-coded selectors and template parts, there needs to be more extensibility so that a theme can indicate how a partial should be rendered, rather than us assuming how it will be rendered. As part of this, we may want to have a collection of theme-specific code that gets loaded when a theme is active. In other words, we can bundle support for the partials unique to Twenty Fifteen, and provide examples for how themes should add support for selective refresh.