Fucking swap files.
Added atbash.
Proper relative imports.
Use comprehensions instead of map.

Fuck functools.
DRY: MonoalphabeticSubstitutionCipher.

Reimplemented our monoalphabetic substitution ciphers in function of a
shared base class, MonoalphabeticSubstitutionCipher, because that's easy.
Stripping is now the user's responsibility.
Added keyword cipher.
Off by one.
Wrote some tests.