Tie into Cucumber event bus but only when used with Cucumber

[mvz]

Aruba should use the features provided by the different test frameworks more. As part of this, when used with Cucumber it should use the Cucumber event bus instead of using a separate bus.

Open questions:

• Is the Cucumber event bus open enough to allow this?
• Do RSpec and Minitest provide something similar, and if not, how can Aruba provide hooks to be run when certain stuff happens?

When this is implemented, we can drop the dependency on cucumber-core, which will be a step towards resolving #402 and #806.