Include update.show.js with NPM package

[RentecTravis]

Is your feature request related to a problem? Please describe.
update.npm.js lets users who don't need the update notice avoid downloading the code to display it. If you want to avoid a request to a third-party URL for the display code, you can use the configuration option jsshowurl. If you just want to self host the same code browser-update uses you need to manually copy and paste the source code for update.show.js from the browser-update repo.

Describe the solution you'd like
Include update.show.js in the NPM package to facilitate self-hosting all JS necessary.

[josselex]

Indeed is seems that update.show,js is not in the package, although it should be there. Thanks for pointing that out. Will be fixed.