extra addons as npm dependencies

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

Start by inspecting the empty extras/ directory, .npmignore, and package.json. Check how the npm package currently handles optional addons and compare that with the git layout. Done means npm users can access optional addons through the same extras/ path without adding them as regular dependencies; the issue notes peerDependencies as a possible mechanism.

Written by the indexing model from the issue text.