Solves Challenge 6

Adds data/6.txt, the challenge's ciphertext testcase.

Adds data/plaintext_corpus.txt, the Project Gutenberg e-book of The
Devil's Dictionary by Ambrose Bierce. This is a public domain work in
the USA. This file is used to build a more thorough frequency table than
the hard coded table I used in Challenge 4.

Reinvents the wheel in a number of places, I could be putting more
effort toward tweaking the solutions from previous challenges but...