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

- Vref of 1.2 V isn't working (may be due to missing setup/hardware)

- This is a patched version to work with Teensy 3.1 (current version on the website doesn't work)
- "Should" work with OS's other than Linux, but hasn't been tested

- Commenting cleaned up
- usb_keyboard_debug is not finished yet in terms of cleanup (needs DEBUB_PRINT #ifdef's)
- Added LoadFile script generation
- Formalized KeyIndex and USBKeys data containers
- Split parts of scan_loop into Macro, USB, and main.c
- Added interface to USB modules
- Added interface to Macro modules
- Added proper Debug module enable/disable

- Include option is currently "hacked" and needs to be fixed
- Builds on Linux, but Mac and Windows needs to be tested
- Loader script generation isn't complete