✨ allow weekly header starting from any number given by user, including negative numbers

[The-Lum]

Hello PlantUML team,

To follow:

• Request - Gantt: Weekscale with weeknumbers? #525

And in order to answer to:

• https://forum.plantuml.net/17852/custom-financial-weeks-in-gantt-chart

Here is a PR in order to:

• allow weekly header starting from any number given by user, including negative numbers

Example [for pdiff]

@startgantt
printscale weekly with week numbering from 11
Project starts the 6th of July 2020
[Task1] on {Alice} requires 2 weeks
[Task2] on {Bob:50%} requires 2 weeks
then [Task3] on {Alice:25%} requires 3 days
@endgantt

Regards,
Th.