Start working on Shell Sort (currently incorrect)

Based on http://mathbits.com/MathBits/CompSci/Arrays/Shell.htm
Add Shell Sort - Need to use more descriptive names

My implementation is kind of different from many sources I found, so I'm
not really sure how it is compared to others. My method makes more sense
to me since I learn this algorithm from this video
https://www.youtube.com/watch?v=M9YCh-ZeC7Y