Add support for bubble charts

This pull request fixes https://github.com/randym/axlsx/issues/253
Only basic options are supported (x/y values and bubble size)

Here is how to add a bubble chart:
```
sheet.add_chart Axlsx::BubbleChart do |chart|
  chart.start_at 0, 0
  chart.end_at 20, 20

  chart.title.text = "Bubble pop"

  chart.add_series xData: sheet["A1:A3"],
                   yData: sheet["B1:B3"],
                   bubbleSize: sheet["C1:C3"]
end
```

Tests were added for basic XML output validation (similar to the scatter tests ones), I'm not really sure how to test anything specific about the bubble chart.