Support ENV

This is a start at solving #1

This is a quick spike at getting ENV support into Atom Runner. This changes the config to allow users to supply env vars that will be loaded before their scripts are run. This is especially important for things like RBENV with Ruby scripts.

#### New Config Format

```cson
'runner':
  'scopes':
    'coffee':
      'cmd': 'coffee'
    'js':
      'cmd': 'node'
    'ruby':
      'cmd': 'ruby'
      'env':
        'PATH': '/usr/local/rbenv/versions/2.0.0-p247/bin:$PATH'
    'python':
      'cmd': 'python'
    'go':
      'cmd': 'go run'
  'extensions':
    'spec.coffee': 'jasmine-node --coffee'

```