Adds support to capistrano multistage completion

Uses `shipit` because [cap is a reserved word](http://zsh.sourceforge.net/Doc/Release/Zsh-Modules.html#The-zsh_002fcap-Module), so `cap` completion doesn't work.

```bash
$ shipit |
production staging

$ shipit production |
deploy                       -- Deploy a new release
deploy:bundle                -- Bundle
deploy:check                 -- Check required files and directories exist
deploy:check:directories     -- Check shared and release directories exist
deploy:check:linked_dirs     -- Check directories to be linked exist in shared
deploy:check:linked_files    -- Check files to be linked exist in shared
deploy:cleanup               -- Clean up old releases
deploy:cleanup_rollback      -- Remove and archive rolled-back release
deploy:finished              -- Finished
deploy:finishing             -- Finish the deployment, clean up server(s)
deploy:finishing_rollback    -- Finish the rollback, clean up server(s)
deploy:log_revision          -- Log details of the deploy
deploy:published             -- Published
deploy:publishing            -- Publish the release
deploy:restart               -- Restart application
deploy:revert_release        -- Revert to previous release timestamp
deploy:reverted              -- Reverted
deploy:reverting             -- Revert server(s) to previous release
deploy:rollback              -- Rollback to previous release
deploy:started               -- Started
deploy:starting              -- Start a deployment, make sure server(s) ready
deploy:symlink:linked_dirs   -- Symlink linked directories
deploy:symlink:linked_files  -- Symlink linked files
deploy:symlink:release       -- Symlink release to current
deploy:symlink:shared        -- Symlink files and directories from shared to release
deploy:updated               -- Updated
deploy:updating              -- Update server(s) by setting up a new release
```

I don't think so. Because if you `gem install cap` it will add [Capistrano executables](https://github.com/capistrano/capistrano/tree/master/bin) in your PATH.