Analyze a local package.json file

[stof]

This is somewhat related to #24

Instead of pointing to a package.json file with a URL, it would be great to allow uploading a package.json file available locally (of course, that would not upload it elsewhere than inside a client-side <input type=file> as everything is done client-side). This would allow visualizing the dependencies of a project even if the project package.json is not available on a public url.