Allow to jump from category to spec

In project I work on we don't modify NSManagedObject's subclasses generated by xcode. All our changes go to `Foo+Bar.{h,m}`. But kiwi spec is located in `FooSpec.m`. With this change this plugin tries to find `Foo+BarSpec.m` first and if no result, then `FooSpec.m` gets a chance.

It doesn't cover back jump: `FooSpec.m` => `Foo+BarSpec.m`. Probably it can be next improvement. for example we can remember the last jump and use it to determine where the next jump should be.