Extend to allow pulling environment vars from global config

Hi,

I have extended the plugin to allow the following usage within the GruntFile. 

Consider this config file:
{  
  development: {
    jsFileName:  'development.js',
    cssFileName: 'development.css'
  },
  production: {
    jsFileName:  'production.min.js',
    cssFileName: 'production.min.css'
  },
  environment: {
    default: 'development',
    environments: ['development', 'production'],
    version: function(){
      return grunt.file.readJSON('package.json')['version']
    }
  }
}

Allows this usage:
sed: {
  js: {
    path: 'build/index.html',
    pattern: '%SRC_JAVASCRIPT%',
    replacement: '<%= grunt.environmentVar("jsFileName") %>',
  },
  css: {
    path: 'build/index.html',
    pattern: '%SRC_CSS%',
    replacement: '<%= grunt.environmentVar("cssFileName") %>',
  }
},  

Which pulls out different values depending on the current environment within strings. very cool :)

Please consider pulling into the main plugin.

Thanks
Jae.