Add 'startAngle' property to pie and donut charts

[YtGz]

New Pull Request

Adds a property named 'startAngle' that determines where pie or donut charts start drawing.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[junedchhipa]

Thank you! Looks good.

[junedchhipa]

Single value pie/donut charts don't work with this.

For eg.,

var options = {
  series: [10],
  chart: {
    type: 'donut',
    width: 380
  },
  plotOptions: {
    pie: {
      startAngle: 180
    }
  },
  labels: ['Team A']
}

This should draw a full circle.

[junedchhipa]

Removing % this.fullAngle from endAngle and prevEndAngle in the commit above fixed it.
I hope that doesn't break other things.