add package.json for publishing to npm registry

[okize]

Fixes #12

@timdown You would still need to npm publish to the registry yourself. Thanks!

[ersimont]

Thanks for this, okize. For anyone else wanting to use this even though the PR hasn't been merged, you can install via npm directly from this PR:

npm install timdown/rangyinputs#pull/17/head

[ersimont]

I'm trying out yarn which does not (yet?) understand pull request URLs. So for that refer to the commit hash instead:

yarn install timdown/rangyinputs#91d0c4e5448a79dc4c47f8771c258c854d690360

[okize]

@timdown any chance of getting this merged? npm install timdown/rangyinputs#pull/17/head doesn't seem to be a viable workaround for newer versions of NPM.