Add support for inline plugins

[kurbar]

Implementation for feature #130

Possibility to add inline plugins to individual charts.

A plugin can be added in the class which extends any of the charts before rendering,

this.addPlugin({
  id: 'kwhWeek',
  beforeDraw(chart) {
    const width = chart.chart.width;
    const height = chart.chart.height;
    const ctx = chart.chart.ctx;
    ctx.restore();
    const fontSize = (height / 114).toFixed(2);
    ctx.font = `${fontSize}em sans-serif`;
    ctx.textBaseline = 'middle';
    const text = '4511kWh';
    const textX = Math.round((width - ctx.measureText(text).width) / 2);
    const textY = height / 2;
    ctx.fillText(text, textX, textY);
    ctx.save();
  },
});

It supports all methods available in the docs http://www.chartjs.org/docs/latest/developers/plugins.html

[codecov]

Codecov Report

Merging #137 into develop will decrease coverage by 7.76%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop     #137      +/-   ##
===========================================
- Coverage      100%   92.23%   -7.77%     
===========================================
  Files           11       11              
  Lines           95      103       +8     
===========================================
  Hits            95       95              
- Misses           0        8       +8

Impacted Files	Coverage Δ
src/BaseCharts/HorizontalBar.js	90% <0%> (-10%)	⬇️
src/BaseCharts/Doughnut.js	90% <0%> (-10%)	⬇️
src/BaseCharts/Radar.js	90% <0%> (-10%)	⬇️
src/BaseCharts/Bubble.js	90% <0%> (-10%)	⬇️
src/BaseCharts/PolarArea.js	90% <0%> (-10%)	⬇️
src/BaseCharts/Bar.js	90% <0%> (-10%)	⬇️
src/BaseCharts/Pie.js	90% <0%> (-10%)	⬇️
src/BaseCharts/Line.js	90% <0%> (-10%)	⬇️