Update npm package

[dariusk]

Can you update the npm package for this library? I was going by the readme on the published package and was confused as to why modifiers weren't working until I came here and saw the updated readme. Thanks!

[MichaelPaulukonis]

Adding the github repo (to package.json) so it points here could be helpful, to avoid situations like this, where people only found the main Tracery repo.

[v21]

Done! (this took a lot longer than it should, sorry)