New accelerometer output program for more convenient testing

Adapted from MPU dmp example.
Outputs combined into one line per frame for ease of reading.
+OUTPUT_RAW_ACCEL
+OUTPUT_GRAVITY
+justMagnitude setting
Changing justMagnitudes boolean determines whether to output just the
magnitudes of accelerations or to output all their components.
The purpose of this is to change the program to fit your particular
testing needs; this particular configuration streamlined mine a lot.
Added acceleration history array and spike/plunge detection functions for better freefall detection

The acceleration spike/plunge approach to free fall detection is a bit
ad hoc, but it avoids the bug isFreeFalling() and even i2C’s
getIntFreefallStatus() functions have where it’s triggered by simply
tilting and holding the ball a certain way.
We still need to play with historyLength and the thresholdPercentages
put into the accelIsSpiking and accelIsPlunging functions in
lightCycle() for best detection.