Update company-arduino paths for v1.8.6

[jbutcher5]

I have adjusted the paths to work for Arduino 1.8.6. I have also added a new environment variable to be used called ARDUINO_AVR_GCC_HEADERS which as the name implies it stores the location of the AVR GCC header files. This is because the path to this location is a bit harder to be figured out. It could be done with just the home path however I thought it would ultimately just be easy to add a path.

Here are where both of my environment variables are set to give a little bit of insight

$ echo $ARDUINO_AVR_GCC_HEADERS
/home/james/.arduino15/packages/arduino/tools/avr-gcc/7.3.0-atmel3.6.1-arduino7/avr/include/
$ echo $ARDUINO_HOME
/home/james/.arduino15/packages/arduino/hardware/avr/1.8.6

These changes will likely only be reflected on *nix based systems and may need to revised for Windows however I don't use windows and don't have much intention of trying to figure it out, I just wanted this awesome package to work for my needs.

The README would need to be updated, I would be happy doing that however unless I see any movement on this PR it's likely not to get done.

Closes #6
Closes #5