Fix subject for #digit
Implement permutations involving underscore

I definitely want to revisit #underscore_permutations, but I'm guessing
I'll know more after I implement it's sibling for handling pipes. So
punt for now.
Generalize character permutation method

Still needs some refactoring
Supply permutations for a character matrix
Add test cases to cover examples in directions
Add pending test cases for corrected_account_number