Describe the workflow you want to enable

Hi all, I wanted to use gray-scale colormap and add labels. july.calendar_plot and july.month_plot allowed me to add labels (a.k.a. ticks, values inside calendar cells) with value_label=True param, but did not allow me to change the color of these labels.

Example image and code attached

july.calendar_plot(data=df_plot.values, dates=df_plot.index, cmap="gray_r", value_label=True, title=False)

Describe your proposed solution

1. As far as I understand the source code, the affected line is below:
   https://github.com/e-hulten/july/blob/master/src/july/helpers.py#L116

july/src/july/helpers.py
> 105 | def add_value_label(ax, cal, value_format):
> ...
> 118 | ax.text(j   0.5, i   0.5, val_format.format(z), ha="center", va="center")

2. Objects of matplotlib.axes.Axes.text class have color or c - color parameter documentation link.
3. We might add additional param responsible for color of values or transfer **kwargs

Describe alternatives you've considered, if relevant

Mayhap there are other ways to fix it

Additional context

Same issue may occur with other colormaps - it is difficult to separate dark colors from dark backgrounds (or to separate same tints, shades, tones in general). Also, color are important both from æsthetical and accessibility points see