Split --cache flag

Without researching, it's not clear what does `--cache` flag do. There are multiple types of possible caches (opcode cache, object cache, page cache), `--cache` name is too broad.

Another thing is that right now `--cache` enables both object and page cache, and there is no way to enable just one type.

This was mentioned in https://github.com/EasyEngine/site-type-wp/issues/50#issuecomment-423437416 by @kirtangajjar but it was not implemented in stable release and I see no other issue that tracks this feature.