Add support for GTVER

[DanBoothDev]

GTVER is a very useful report for firmware/hardware versions. I added it in #14 but only implemented GL300M devices.

We might also want to add support for more items in devices.h such as:

int device_type;    /* String representation of the device type e.g. gl300m */
int ver_fw;            /* HEX firmware version MAMI for MAjor MInor */
int ver_hw            /* HEX hardware version MAMI for MAjor MInor */

This is something that I could happily undertake and submit a PR when ready.

[jpmens]

I think that's a good idea, @DanBoothDev, and we'll happily entertain your PR when you're done. Thank you.

[jpmens]

Closed via #16