Move rxjs to peerDependencies to allow custom versions usage

[kctang]

I am getting "TypeError: You provided an invalid object where a stream was expected. You can provide an Observable, Promise, Array, or Iterable." in my application and I traced it to this library's dependency on rxjs.

While my application does not use listr directly, it has webpack-cli as a devDepdencies. And webpack-cli depends on listr.

Hope you can make this change so I don't have to 'workaround' the issue.

Also, as mentioned here, rxjs should be a peerDependencies in library modules.

Cheers!

[gpetrov]

@SamVerschueren please get rid of the rxjs dependency this brings 16MB and 3.5K files to the node_modules - way too much!