Allow use of @apply in gcss

[vendion]

I'm trying to use Gcss with polymer and some of the components require the use @apply in the CSS. Currently gcss strips these tags out when compiled. It would be helpful if when compiling gcss would ignore such tags.

To see an example of how these are used see https://github.com/PolymerElements/iron-flex-layout/blob/master/demo/x-app.html

Example gcss file

html, body
  height: 100%

body
  margin: 0

:host
  @apply(--layout-horizontal)
  @apply(--paper-font-body2)

[nav]
  @apply(--layout-vertical)
  width: 200px
  background-color: var(--paper-grey-300)

Generated css

html,body{height:100%;}body{margin:0;}[nav]{width:200px;background-color:var(--paper-grey-300);}