Vertical white space is not configurable

[leecbaker]

There's a fixed 1 unit of white space added above and below bar codes. I'd love to be able to adjust this; seems like it would be great as a writer option.

Should be simple and quick to implement.

[marcoaaguiar]

The extra space is added due to the 2.0 offset in line

python-barcode/barcode/writer.py

Line 116 in 83a0b4e

height = 2.0 + self.module_height * number_of_lines

Because of the offset, there is a hardcoded half offset in

python-barcode/barcode/writer.py

Line 205 in 83a0b4e

ypos = 1.0

I believe this could be a new Writer option (extra_vertical_space) so I don't break backward compatibility.

Edit: @WhyNotHugo would this solution be accepted?