On Windows, check that ARDUINO_DIR is relative

[sej7278]

As reported in #45 and #94 (and in README.md) on windows we need to use a relative path for ARDUINO_DIR.

Therefore we should probably check that if CURRENT_OS == WINDOWS then ARDUINO_DIR must be a relative path or we should exit with a warning to the user, or try to do something clever like figure out what the relative path should be based on the absolute path provided.