More incremental improvements

[ElectricRCAircraftGuy]

Do a ton more incremental improvements to the software formatting

• Change #define section from two #ifs to #if #else
• moved global variables to one per line
• changed some types to use stdint.h types
• added a few comments and some doxygen

Also:

• added a changelog
• started tracking a version number
• added a checkbox to the PR template to ensure the version number is incremented each time the code is changed

Checklist:

• [x] I have run the code formatter with ./run_clang-format.sh (see main README.md for details on how)
• [x] I have built the code in the Arduino IDE (or with the command-line tool) and pasted the build output below

Arduino IDE build output:

20200422-0136hrs (git hash 222e6966211a877936bd2962692c0c57bd665e2f; branch more_incremental_improvements) (IDE 1.8.12) (Arduino Nano)
Note: memory usage went up because we are now storing and printing the version number at startup.

Sketch uses 18318 bytes (56%) of program storage space. Maximum is 32256 bytes.
Global variables use 1074 bytes (52%) of dynamic memory, leaving 974 bytes for local variables. Maximum is 2048 bytes.

[nimrod46]

Ahh, great job as always! I will merge your PR instead of just accepting them from now.