Provide and API/SPI to solve backward compatibility

For extension relying on Drone 1.2.x, migration to 2.0.0 should be smooth. Identify what API/SPI needs to be supported based on existing extensions:

These are at least:

• Graphene 2
• Warp
• JBehave
• CukeSpace
• APE
• SauceLabs

Note, likely we won't be able to support backwards compatibility 100%. We rather need to make sure major extensions using Drone stay compatible or provide a very easy migration path, e.g. Drone compatibility package.