Closes https://github.com/CocoaPods/CocoaPods/issues/2388

* lookback-pods-by-config: (45 commits)
  [UserProjectIntegrator] Warn about xcconfig override after every installation /2
  [UserProjectIntegrator] Warn about xcconfig override after every installation
  [AggregateTarget] Add #user_targets
  [AggregateTarget] Documentation cleanup
  [Bundle] Update
  [PodTarget] Ensure that subspecs are on the same configurations
  add a space to the override warning
  [Docs] Some small fixes.
  [Changelog]
  [Integration] Disable xcproj in Xcodeproj
  [Project] Coherce preprocessor definitions to an array
  [PrivatePodXcconfig] Add inherited to GCC_PREPROCESSOR_DEFINITIONS
  [Project] Add preprocessor definition for build configurations
  [TargetEnvironmentHeader] Refactor
  [AggretateTarget] Expose specs by build configuration
  [PrefixHeader] Style fixes
  [Xcodeproj]: Fix issue for libraries with a space in the name
  [Examples] Update integration
  [Installer] Use downcase to validate configurations
  [XCConfigIntegrator] Downcase the name of the build configurations
  ...

Conflicts:
	CHANGELOG.md
	Gemfile.lock
	lib/cocoapods/installer.rb
	lib/cocoapods/target/aggregate_target.rb
	spec/integration.rb

Also includes some small fixes here and there

[Plugins] Added support for hooks registratrion