initial implementation of the undo command; serializes/deserealizes bitmap pixels to keep snapshots for the undo
implements the undo feature
simplifies app exit; removes unnecessary Command attributes
refactors bitmap fill method to read better
changes bitmap clear method to iterate current pixels matrix instead of creating a new one
command parse method refactor to read better
bitmap to_s method now returns new when pixel matrix is empty to catch on cli start error handling
little improvement on app aesthetics
adds an useful README file