Add support for hpgl export about vsketch HOT 2 CLOSED

Hi
i can work on?
it would be great to tell me more about this issue

from vsketch.

Vsketch.save() currently always generate SVG (using vp.write_svg()). The goal would be to support HPGL export as well, using vp.write_hpgl(). The way I see things would be:

• add a new format parameter (default to None)
• attempt to infer desired format from file extension (if available, raise error otherwise)
• add tests to validate that the generated output is indeed in the expected format
• update the documentation and/or examples

from vsketch.