Make it possible to import parts of your configuration from other files

[d-fischer]

Fixes #292

Still needs to complete the requirements list, as well as some tests for a few of the requirements.

Requirements

• The $import special key should instruct the loader to load the given file(s) using the same explorer the consumer used, and merge your own configuration into it
  • Tests
• Your own configuration should take priority over any imported file(s)
  • Tests
• $import should be able to contain a string directly, or an array of strings
  • Tests
• When importing multiple files, the last one takes priority
  • Tests
• Nested importing should be possible, i.e. you should be able to $import a configuration file which itself uses $import
  • Tests
• There should be a circular dependency check - loops of any size should lead to an error being thrown
  • Tests
• You should be able to $import files of a different format than the one you're using
  • Tests
• There should be a configuration option for the consumer that determines how to handle arrays on the same object key
  • Tests