Did think about a 24 unit approach, however we concluded it was not the way forward as the unit widths at 24-points would be ok at 4K, but at sub-4K they would be negligible, and the developer would need to work in multiples.
Well the 12-unit grid is normally chosen based on being divisible by [1,2,3,4,6,12]. 16-unit is divisible by [1,2,4,8,16], but I see that you added a "thirds" class to deal with that. 20-unit could be an option as it covers divisibility by [1,2,4,5,10,20] but also critically ignores 3 column. 24-unit (the product of the first four positive integers) is divisible by [1,2,3,4,5,6,8,12,24], so it would naturally allow the most flexibility at the cost of very granular 1/24 elements.
Thee "pull-left" and "pull-right" classes were added into v2.2 (in the Github repository) in addition to the similar "float-left" and "float-right" classes.
These additions have been merged into the Master branch this afternoon.
The example site is running on v2.1 which uses the float-left and float-right classes.