Fixes #265

Provide more comprehensible alternative for tuning entropy checking.
This is applied consistently across all target character sets, and stated
in a way that is slightly easier to understand ("higher means more
likely to flag a given string").

The older `--b64-entropy-score` and `--hex-entropy-score` options are
marked as deprecated but retained for backwards compatibility (and they
override `--sensitivity` if used together with it).

Use `--entropy-sensitivity` instead of `--sensitivity`

...in response to review comments

...and use them instead of private members

Do everything from scratch instead of storing explicitly. This is a PITA
because you can't combine `@property` and `@lru_cache()` and skipping
the caching would be a killer.

* Consolidate common code for entropy limit back into a single method,
  and rework properties related to it so they are cleaner.
* Invert sensitivity scale; adjust math and doc to match. It's still
  weird but aligns more closely to the underlying entropy metric.