see lib/fancy_buff.rb for usage

run the minitest specs:

$ bundle exec ruby -Ilib:test lib/**/*_test.rb

• add a cursor (not visual, but a char index)
• inserts/deletes should move marks, possibly removing them in a delete
• inserts/deletes should move selections, possibly removing some/all of them in a delete
• add yanks (creating of dynamic char ranges as selections)
• add highlights (reusable visually highlighted ranges, which maybe is just a selection that some other tool will highlight visually?)
• consider whether or not everything should be a range
  • marks can be single or multi-character marks
  • selections are already ranges
  • yanks are a special kind of range that is created when copying something
  • highlights are just selections displayed visually, which is some other tool's job