Stdout and Stderr passed as arguments to the afterDeath closures

## The problem
Whenever using `afterViolentDeath` or `afterNaturalDeath` it's not possible to access `stdout` and `stderr` from within the closer triggered by `afterViolentDeath`. When a process fails and we trigger a an action with `afterViolentDeath` we usually want to know *what* happened and log down the error.

## The fix
This PR attempts to fix this. New signature for `afterViolentDeath` is `function($status, $stdout, $stderr)`, for `afterNaturalDeath` is `function($status, $stdout)`. I didn't really see a reason to make `stderr` available in `afterNaturalDeath`.

## Changes
- New signature for `afterViolentDeath` and `afterNaturalDeath`
- Added related tests
- Updated `README`