Refactor config-changes.js, p20. Rebase.

Rebased on top of current master where another caching mechanism for iOS project files
was introduced in commit 1655ada. Keeping both of them for new because for now
ConfigKeeper doesn't help to speed up adding multiple plugins. Added a TODO
comment to remove caching from ios parser once multiple plugins can be
installed with a single instance of PlatformMunger.