v1.8.0

• v1.8.0 supports Chart.js 2.8.0
• Support ticks.reverse scale option
• Support pointRotation and borderSkipped arrays
• Support chartjs-plugin-datalabels 0.6.0
• Support chartjs-plugin-zoom 0.7.0
• Reduce the code size by removing the ported code from Chart.js
• Add a reverse sample