Add after_do to microrb projects

* focussed on adding callbacks to methods and doing that well
* no monkey patching!
* no runtime dependencies!
* small code base - simplecov reports less than 60 relevant loc
  and overall it's 120 locs with docs, blank lines etc.
* @chastell uses it :)
* used in shoes4 to handle redrawing and not litter that concern
  everywhere

I'm a bit unsure about the tags. It's Aspect Oriented Programming/callbacks, imo none of the others fit so I just went with utils :)