Add title, subtitle and footnotes option

[jkrumbiegel]

This could be done before too by using Label manually on the figure resulting from draw, but this is much more convenient:

spec = pregrouped(
    fill(1:5, 6),
    fill(11:15, 6),
    [reshape(sin.(1:25), 5, 5) .+ i for i in 1:6],
    layout = 1:6 => nonnumeric) * visual(Heatmap)

draw(
    spec;
    figure = (;
        title = "Numbers in square configuration",
        subtitle = "Arbitrary data exhibits sinusoidal properties",
        titlealign = :left,
        footnotes = [
            rich(superscript("1"), "First footnote"),
            rich(superscript("2"), "Second footnote"),
        ]
    ),
    axis = (; width = 100, height = 100)
)