Need way for other libraries to get LMIC config in scope

[terrillmoore]

Other libraries (such as arduino-lorawan) need to get the configuration of the LMIC, which is painful; they do this to get the configured region and so forth. But the name of the lmic library will change based on how the user installls it. So.. we need a new header file, `<arduino_lmic_user_configuration.h>, that brings in the proper stuff.

This needs to go out as an urgent patch release.

See mcci-catena/arduino-lorawan#65