code: function parseCss (ctx, done) {
    var result
    try {
      result = postcss()
        .process(ctx.src, {
          map: {
            sourcesContent: true,
            annotation: false,
            prev: ctx.map
          },
          from: ctx.filename
        })
      ctx.src = result.css
      ctx.map = result.map.toJSON()
      done(null, ctx)
    } catch (e) {
      var msg = e.message
      var ext = extname(ctx.filename).slice(1).toLowerCase()
      if (ext !== 'css') {
        msg += ' (Try to use appropriate parser for ' + ext + ')'
      }
      done(new Error(msg))
    }
  }
docs: Default source parser for css source
  
  @param {Object}   ctx
  Cssy context object with source
  
  @param {Function} done
  Async callback