Allow for alternate expected plaintext frequency distributions across FD

[unicornsasfuel]

Currently, large parts of Cryptanalib and FeatherDuster assume that the expected frequency distribution for the plaintext of the target samples is American English, as generated for and distributed with Cryptanalib.

Since we provide functions for end users to generate their own frequency data compatible with functions in Cryptanalib, that data should be usable in functions which rely on frequency data.

(Note: If you work on this issue, please add milestones as functions or modules are converted to allow custom frequency data.)