Update to Polymer 2, clean code and add auto build script

[myfrom]

On the beggining, I would like to say sorry that I didn't split this to smaller PRs but this repo hasn't been maintained for a while and I needed all of the changes to test their functionality.

What does this PR change?

Added support for Hybrid mode

This way developers can use this element in both Polymer 2.0 and 1.x This fixes RubaXa/Sortable#1237

Added automatic build script

This helps the element to stay up to date with latest Sortable version. The framework sometimes changes options that it accepts, now running build.js will automatically add those options to the elements as parameters so they an be passed, I've described that deeper in CONTRIBUTING.md

Updated for Sortable#1.6.0

Which also confirmed my script works 😃 I didn't use 1.7.0 because it's not available on Bower 😢

Changed .gitignore

This is really small change, I added a few VSCode files to be ignored and also removed ones like mock.png and /test which should not be ignored.

cc @RubaXa PS: This repository hasn't been updated for a while, I guess it's because you might have no time for it. If it would be possible I could take care of this repository for you. I often work with Polymer, have plenty of free time and I'd love to help 👨‍💻️

[ghost]

Can't wait for this to be merged! Awesome work!

[myfrom]

@dcbrwn Thanks 😄

You actually reminded me that I should update it to Polymer 3 as well 🤔 I'll look into it probably after I finish my current project