Allow generation for double-sided flaps

This allows the generation of double-sided flaps, ideal for laser cutters/printers.

This PR adds the following arguments to generate_fonts.py:

--double-sided: Generate a double-sided version of the flaps, for double-sided printing and laser-cutting.
--bleed: Bleed the letters. This allows some margin for error during double-sided printing and laser-cutting.
--start-row: The row to start rendering. Combined with the --row-count, this allows pagination of the flaps.
--row-count: How many rows to render.

Demo:

Sample of Generated Flaps:

scottbez1 / splitflap