title: Can I specify optional module dependencies in npm package.json?
body: <p>I have a service where I want to allow users at install-time to specify which persistence engine to use, i.e. file-based, MongoDB, or Redis, and I'm looking for some npm magic where you only download the necessary modules (none, mongodb, or redis, respectively).</p>
  
  <p>Is this possible?  I can't find any options other than defining dependencies and devDependencies in package.json, and that's not appropriate for this.</p>
  
  <p>Note also that while the mongodb and redis modules may be relatively small, consider an alternate case where you may optionally need Java for RMI communication.</p>
  
  <p>Thanks!</p>