Add minor gridlines

[sorawee]

Per @schanzer's request.

For example, for the program:

include chart
import color as C

fun some-fun(x): num-sin(2 * x) end
a-series = from-list.function-plot(some-fun)
  .color(C.purple)
a-chart-window = render-chart(a-series)

a-chart-window
  .x-axis("this is x-axis")
  .y-axis("this is y-axis")
  .display()

In CPO, it displays:

With this patch, it displays:

Here's another example for the 10x + 7 graph:

vs

Notice that the number of lines in the minor grids are not the same (across chart / across axes). It might be possible to do that by using a right Google Charts configuration, but that's future work.

[blerner]

I'd be happier if the minor-gridlines were a builder method that we could toggle on and off, rather than something permanently on. E.g. something like a-chart-window.show-minor-gridlines(true/false). These gridlines look very busy to me, and I'd imagine several folks would want them configurably off. (I don't mind if the default is on, or if @schanzer can easily tweak the background code to turn them on, assuming that students won't write the chart-window code themselves.)

[sorawee]

I can make the change tonight, unless anyone takes over the PR.

[jpolitz]

I'm going to merge this because it looks good, addresses Ben's feedback, and I expect it to pass on travis-ci.com after the merge. Thanks @sorawee !

[jpolitz]

Up now on code.pyret.org