Start using release tags & link to unpkg

[cwervo]

Hi!

Might I suggest two changes that would help with the stability of this project? It's a great effort, and I plan on using it more often in my work, so I'd love to give back to it! Two suggestions wrapped up in one "maintenance" topic:

1. Start using Github releases, so you can point to rawgit of tags, that way even if master changes/breaks, people can always have a reliable resource to link older versions of this file, and you don't have to worry because Github will host the file(s).

2. Start linking to unpkg for the distribution file. npm already supports versioning, and unpkg gives a reliable source of truth, whereas Rawgit (while historically stable) has _no guarantees about uptime & reliability. It'd mean changing the way the script gets compiled, I believe, because the current Unpkg link isn't usable by a vanilla JS project, but I've seen other A-Frame projects link to it like super-hands so it's certainly possible, and v reliable!

[MartinKolarik]

You can also link to jsDelivr (the fastest OSS CDN) without changing anything: https://cdn.jsdelivr.net/npm/[email protected]/dist/aframe-ui-widgets.min.js