Publish emsdk on npm + integrate with npm

Right now it's kind of kludgy to create npm packages that are built from C sources using Emscripten. Normally, when doing a local 'npm install', the devDependencies for the package are pulled from npm. But if Emscripten is a development dependency, people need to "manually" install emsdk, activate the right version, etc. It would be great if Emscripten could also be pulled from npm to 1) make this less kludgy 2) enable automatically activating the Emscripten/node/llvm versions required for the project in question, for example based on additional info in the package.json file.