Official migration to Polymer 2.0

A little late, but here it is! You can now simply install Highcharts the normal way, and the default target will not be polymer 2.0.

If you're still on Polymer 1.0, you can use the branch polymer-1.0:

bower i highcharts-chart#polymer-1.0

Feel free to report an issues, since I have not been able to personally dedicate as much time to this library as of late as I used to.