help wanted
描述
The extras/ directory is simply empty in the npm package (via .npmignore). To provide an npm user the same seamless experience as is provided with git, it would be cool if the extras/ directory would be populated with symlinks to a node_modules path where those extra plugins are installed. This way the same path will work whether impress.js was installed via git or npm.
Note that since extra addons are optional, they should NOT be listed as dependencies in package.json. However, it seems to me they should be peerDepencies: https://docs.npmjs.com/files/package.json#peerdependencies