Add more options to apipie:cache

This patch adds ```cache_part=[index|resources]``` and ```OUT=/some/dir``` options to rake task ```apipie:cache```.
In our enviroment the API is quite big and with the localization it takes a lot of time to generate the cache. With these options I can pre-generate part of the cache during the build time to save some time during installation.
I used the same convention I found in other apipie tasks. I'd prefer to have them consistent, should I rename them all? I'd be concerned about backward compatibility though.