Add support for Rails Cop

```rubocop``` needs to be ran from the project directory where the Rails app lives to work.  Therefore, if ```rubocop``` is used then change the CWD to the project path if the Rails flag has been set.

To enable this add ```rails: true``` to your ```config.cson``` e.g.
```
'atom-lint':
  'ignoredNames': [
    'tmp/**'
  ]
  'rubocop':
    'rails': true
    'ignoredNames': [
      'db/**'
      'spec/**'
    ]

```

All specs pass in Atom (0.118.0 using React Editor). Using the ```rake``` command one fails, but it also fails in master, hopefully a separate issue.

```
CommandRunner
  run
    when environment variables of the login shell cannot be fetched
      it runs the command with the current env
```

Since whether Rails cops should be enabled or not is project-specific thing, I think it should be enabled via [`RunRailsCops` directive in `.rubocop.yml`](https://github.com/bbatsov/rubocop/blob/v0.24.1/config/default.yml#L16-L18) rather than this global `atom-lint`'s configuration.