[pod] Whitelist pods for configurations

An option to only link a specific pod to the target when building in a specific configuration.
The exact syntax is still TBD, pending discussion in https://github.com/CocoaPods/CocoaPods/pull/1668
which implements the corresponding functionality.
[pod] whitelisting: code style and specs
[TargetDefinition] Coerce configuration names from user input to strings.
[TargetDefinition] Return list of all configurations for which pods have been whitelisted.
[TargetDefinition] Make predicate method more Ruby-ish.
[TargetDefinition] Also take singular :configuration for whitelisting.
[TargetDefinition] Cleanup docs.
Merge remote-tracking branch 'origin/lookback-pods-by-config' into lookback-pods-by-config-2

* origin/lookback-pods-by-config:
  [TargetDefinition] Cleanup docs.
  [TargetDefinition] Also take singular :configuration for whitelisting.
  [TargetDefinition] Make predicate method more Ruby-ish.
  [TargetDefinition] Return list of all configurations for which pods have been whitelisted.
  [TargetDefinition] Coerce configuration names from user input to strings.
  [pod] whitelisting: code style and specs
  [pod] Whitelist pods for configurations

Conflicts:
	lib/cocoapods-core/podfile/target_definition.rb
	spec/podfile/target_definition_spec.rb
Style: fix