Feature request: specify algorithm for fret scaling

[Tails]

Someone I know... is very fond of Guitar Pro 4 and likes to use the fretboard image as a background.

It would be cool to be able to pass the fret scaling function to make the scales fit. As well as pass in a background image of course, though that is relatively easy to do with CSS.

Thanks for the awesome library btw! Works really well, and the functionality is quite exhaustive so this must have been quite some work! Much respect.