allow project to redefine rubocop settings

[shagabutdinov]

Hey!

It would be great if user would be able to define rubocop settings on per-project base (when using embedded projects functionality of sublime). In that case user can define something like:

{
  "folders": [
    {
      "path": "/home/leo/src/myproject",
    }
  ],
  "settings": {
    "rubocop_config_file": "/home/leo/src/myproject/.rubocop.yml",
  }
}

In project configuration and use custom rubocop settings file or command for this project.

Also rubocop_chdir option added to configuration:

{
  "folders": [
    {
      "path": "/home/leo/src/myproject",
    }
  ],
  "settings": {
    "rubocop_command": "bundle exec rubocop",
    "rubocop_chdir": "/home/leo/src/myproject",
  }
}