Add $__task so available in @setup

[arwinvdv]

Add the variable $__taskName so you can get the current task/storyname in the @setup directive. Now you don't have to specify extra variables when creating multiple stories. Example:

When running envoy run deploy, that you can get deploy. And when you run envoy run deploy_backend you can get deploy_backend

@servers(['localhost' => '127.0.0.1'])

@setup
   $releaseDir = $baseDir . '/releases/'.date('YmdHis');

   if($__taskName === 'deploy_backend') {
      $releaseDir =  $baseDir . '/current';
   }

@endsetup

// Complete deploy, create new folder and check project, run composer, npm etc.
@story('deploy')
   git_clone
   commontask_1
   commontask_2
   ....
@endstory

// Only update backend, pull backend in current dir and restart application, migrations etc.
@story('deploy_backend')
   git_pull
   commontask_1
   commontask_2
   ...
@endstory

Fixes #253