Firmware 0.2.0

• Host pairing (protocol version 2). The bridge only accepts input from paired
  hosts; pairing and every settings change are confirmed with the BOOT button.
  Up to four pairings; holding BOOT for 10 s removes all of them.
• The flasher site can list and remove pairings and turn the requirement off.
• New LED states for "waiting for BOOT" and "BOOT held".
• Hosts need libvirtualhid with protocol 2 (Sunshine esp32-bridge branch from
  this release on).