Introducing parallel execution of tasks

[paslandau]

Q	A
Branch	seventies
Bug fix?	no
New feature?	yes
BC breaks?	no (hopefully?)
Deprecations?	no
Documented?	no (poc => please feedback)
Fixed tickets	#584,#581,#580,#394

tl;dr

• made some bigger changes to allow parallel execution of tasks
• currently only works for external tasks - internal tasks like the linting would have to be exposed as a command line executable
• introduced e2e/integration tests (because I don't know PhpSpec :D)
• for this "test", I added some depencies to composer.json and added a grumphp_parallel.yml,
  so you can run composer install && vendor/bin/grumphp run --config="grumphp_parallel.yml"
  to see the thing in action. I also add verbosity output to see the actual command, see
  vendor/bin/grumphp run --config="grumphp_parallel.yml" -vvv (messy, but helpful for understanding a task)
• this is a POC and I realise that I might have a somewhat different coding style, sorry for that.
  I don't cling to any of the implemenation, I just wanted to get something "runnable" to get an impression
  of how a parallel execution might look like and I think that this might actually be a pretty nice
  extension to grumphp :)

See https://github.com/paslandau/grumphp/blob/parallel/CHANGELOG.md for further info.

[veewee]

Parallel tasks are implemented in #741. Already borrowed the good ideas from this PR already :) Thanks!