Support positioning custom processors relative to any existing processor

[DerManoMann]

Relates to #667

Summary

• Custom processors in scanOptions.processors can now specify where they should be
  inserted in the pipeline using an array form: ['class' => ..., 'after' => ...]
• Plain string/instance entries retain backwards-compatible behavior (inserted after BuildPaths)

Example

'processors' => [
    // Inserted after BuildPaths (default, backwards compatible)
    MyProcessor::class,

    // Inserted after a specific processor
    ['class' => AnotherProcessor::class, 'after' => AugmentSchemas::class],
],

[coveralls]

coverage: 99.405% (+0.02%) from 99.39% — DerManoMann:processor-positioning into DarkaOnLine:master