feat: allow adding heading text to the fretboard diagram

[FUSAKLA]

Hi @dmpayton, I really like the library, thanks!

I was just missing the possibility to add heading to the diagram.
I want to add it to a songbook, so it can be printed out.

Example result:

from fretboard import Chord
style = {
    "drawing": {
        "font_size": 8,
        "height": 150,
        "width": 100,
        "spacing": 15,
    },
    "marker": {
        "radius": 5,
        "stroke_width": 1,
    },
    "heading": {
        "height": 15
    }
}
chord = Chord(positions='xx0232', fingers='---132', name="D", style=style)
chord.save("D.svg")

So if you are willing to accept this I'd be glad to avoid maintaining own fork :)