- Pressed key will remain on the same layer until released
  regardless of the layer changes that happen in the meantime

Check for dfu-util and screen before using them. Also aborts if
teensy_loader_cli compilation fails.

Uses the type builtin to determine if a command exists. See here:
https://gist.github.com/sdelmas/7377271

- Fastload option (allows for remote flashing)
- Autoscreen option initiates screen after successfully loading the firmware