Clean up API about child-process

[loilo]

Having a Promise-based helper as in (4) would be really useful.

A thing that should be baked in: Such an interface would offer the great opportunity of combining $promise->cancel() with $process->terminate() for really easy premature elimination of long-running processes.

The only issue with 1) would be that there is no interface to type declare against. With #74 in mind an interface would possibly be required anyway, so the problem of mocking would be solved. To be clear, the "low level" API of directly using a Process should still exist, even when a launcher should be added.

However interfaces should exist so that the implementation can be arbitrarily replaced or customized, without introducing any dependencies or adaptions in code.

I'd like to pick up 1) + 4) (Launcher concept), are there any specific helper methods that need to exist?

On my list currently:

• launch ("builds" and starts the process)
• buffer (launch + buffer stdout stream)
• stream (should it only capture stdout?)