Added transform option

[gcphost]

If transform option is defined the transform settings object will be
merged with the settings defined allowing users to over-ride or append
new settings.

new BabiliPlugin({
    comments: false,
    transform: {
      presets: null,
      plugins: [
        require("babel-plugin-minify-dead-code-elimination"),
      ]
    }
  })

[boopathi]

Closing this in favour of babel/minify#162