v0.20

Release Notes:

• gw write: New options --erase-empty and --double-step
• gw update: Fix COM redetect on Windows running Bitdefender
• Support version 2 of Ant Goffart's F7 Plus
• SCP: support legacy single-sided SCP images
  • Automatic import; Export via ::legacy_ss suffix
    • eg. gw read --single-sided my.scp::legacy_ss
• SCP: Support SCP image files generated by FluxEngine