Properly destroy chartjs objects

[LinusBorg]

Currently, the chartjs object is not properly destroyed (cleaning up event listeners etc) when the component is destroyed.

This pull request saves the chart instance on the component and adds a destroy() hook to the component that calls the chart instance's .destroy() method to ensure proper cleanup.

methods: {
    render (data, options = this.options) {
      this._chart = new Chart( // save instance on a property of the vm
        this.$els.canvas.getContext('2d'), {
          type: 'bar',
          data: data,
          options: options
        }
      )
      this._chart.generateLegend()
    }
  },
  beforeDestroy () {
    this._chart.destroy() //destroy chart when component is destroyed
  }