Add ability to specify custom rake command

I found it useful to add ability to customize rake command - especially when you use `zeus` in your project.

I'm still working on that and here is list what should be improved before merge:
* ~~at the moment setting custom command to `zeus rake` will produce rake string as `zeus rake rake (..)` - need to fix that~~
* ~~add ability to set custom rake task by specifing custom command, like cmd being set to `zeus rake db:version` will produce rake string as `zeus rake db:version db:migrate db:test:clone`~~

@glanotte I'm curious about what you think of it and if you have any questions/suggestions