feature - allow speedyloader to load firmware from a file

[mike501]

This would enable developers to create firmware and share it with users far easier. This would reduce the barrier to people testing new firmware as compiling puts people off. All the developer would have to do is share the binary file.

[noisymime]

This has now been added. It requires testing though and there are still some error checks to be put in place (Eg selecting a mega2560 firmware file and a Teensy 3.5 board WILL cause a problem), but it should be working and will be included in the upcoming release.