0.1.0-20190914

• use gcc-8.2.1 to build firmware
• contributed by cho45
  • add screen capture command (use script ./python/nanovna.py -C)
  • add trace format of real/imag/impedance/reactance
  • add battery indicator (needs install D2. any small diodes are suitable. 1N4148WS or B5817WS is the best. see https://lowreal.net/2019/09/09/1)
  • add version info screen
  • add menu and command to enter dfu mode
  • add menu item to calibrate touch screen and test it
  • optimize line drawing
  • reduce sram usage