[#7] Allow specifying handlers in route configuration

- Routes may specify a "handler" key; if present, and a callable and/or
  class that resolves to an invokable, it can be mapped to the
  dispatcher. Additionally, this means that the Dispatcher argument to
  the constructor becomes optional, and we will create one if none is
  passed.
- If a handler is _already_ in the Dispatcher, the handler in
  configuration will be ignored.
[#7] Documented usage of "handler" configuration
[#7] CS fixes (trailing whitespace)
Merge branch 'feature/7'

Close #7