add parallel, race, and fallback support

##### Parallel
Add a `parallel` property with middleware spec definition object to have middleware executed in parallel.

##### Race
Add a `race` property with middleware spec definition object to have the first completing middleware complete fulfill the placeholder and advance the middleware chain.

#### Fallback
Add a `fallback` property with middleware spec definition object (and priorities) to execute each moddleware in serial until one completes successfully.


TODO: Docs.