Refactor OpenFiles()

  * removes named return values
  * renames function to be consistent with standard library
  * replaces branched conditional with guard statements
  * return empty slice on error
Remove named return value in ShuffleRunes()
Tidy Scramble()
Combine Scramble() and Randomize()

  * combines both modes into a single function
  * changes Scramble() signature: adds new `random` parameter to
    specify that letters should be replaced with random letters