Added an optional css class to the x/y axes

[dgwyer]

This is my first PR! 🙂

Related. #6

This PR introduces optional CSS classes for the x and y axes.

Plot.plot({
    x: {
      axisClass: true
    },
    y: {
      axisClass: true
});

If axisClass is not defined or is false, then no CSS classes get added to the plot. I kind of like the current clean tree of attributes that Plot creates by default. So I considered maintaining this and only adding CSS classes for axes if specifically opted in.

If axisClass has been defined then it inserts the following classes. Note: Only the classes for the y-axis are shown below, but it's exactly the same for the x-axis if CSS classes are enabled.

<g class="y-axis" aria-label="y-axis" aria-description="↑ Close" transform="translate(40,0)" fill="none" text-anchor="end" font-variant="tabular-nums">
  <g class="tick" opacity="1" transform="translate(0,358.4133255882662)"><line stroke="currentColor" x2="-6"></line><line stroke="currentColor" x2="580" stroke-opacity="0.1"></line><text fill="currentColor" x="-9" dy="0.32em">60</text></g>

  ...

  <text class="y-axis-label" fill="currentColor" font-variant="normal" transform="translate(-40,20)" dy="-1em" text-anchor="start">↑ Close</text>
</g>

FYI - I've not added any docs for this PR yet. If it looks like it will be accepted then I'll update it of course.

[Fil]

Thanks for the contribution. I'm closing because we're probably going to revisit axes to change them to marks (see #1000), and we're also thinking about a generic className option that one could use on any mark (see #253, which would have to be rewritten as a "common option"). These two large generic changes would end up solving this.