Add Corner Radius Config Option for Bar Chart

[GeorgeBerdovskiy]

We currently support some very nice bar charts, but styling options are quite limited. This goes against our goal of complete control over chart appearance! Let's work towards that goal by adding a configuration option that specifies how rounded the corners should be. Check out the image below for guidance.

Right now, the configuration argument looks like this -

@configuration = {
  axes: {
    horizontal: "Lines of JavaScript",
  }
}

Change the bar_chart helper code to accept arguments that also look like this -

@configuration = {
  axes: {
    horizontal: "Lines of JavaScript",
  },
  corner_radius: "5px"
}

However, remember that this should be an option! In other words, if the user doesn't specify a corner radius, use some default. Smooth sailing ⛵️

[sarayumummidi]

I can take a shot at this!

[GeorgeBerdovskiy]

Closed by #43