Compile option for language and units

[geofrancis]

I tried to do an English version with miles as the units as I am in the UK but my coding skills aren't that good, managed to translate the menus but I never got around to changing the math to miles I just changed the units and changed the wheel size by 0.63 to compensate. It would be a much better solution to have this integrated into the main branch as a compile option and would open it up to a lot more users.

https://github.com/geofrancis/Arduino-Pedelec-Controller/blob/b362867af142b6028e2122b7b07b4844ce74a9d9/Arduino_Pedelec_Controller/menu.cpp