Keymap in eeprom

Hi Hasu,

I tried to write a keymap-in-eeprom feature that make the firmware can read keymap from eeprom instead of flash at runtime. 

So with this, users can change their keymap settings by only reflashing something in the eeprom. And I also wrote [this](https://github.com/kairyu/tkg) to make it easier to generate an eep file with proper keymaps in it. [Demo](http://www.enjoyclick.org/tkg)

This is only an optional feature and will not affect others, I think.

Best regards,
Kai