Add converse.js as NPM dependency to package.json

Instead of using `git submodules`, you can then use `npm link
converse.js` to link in an existing checkout of converse.js

Note, before running `npm link converse.js` inside the
xmpp-chat-component checkout, you'll need to run `npm link` inside the
converse.js checkout.

Doing it this way removes the need to check out converse.js again
(via git submodule) when you already have a checkout of converse.js
somewhere and also makes the NPM dependency on converse.js explicit.