Generator refactoring

[woodruffw]

KBSecret's generator interface is currently pretty inflexible -- all generators take a format and length, and the current Generator class just uses SecureRandom.

Ideally, generators would be more like record types -- extensible by users, and pluggable.

A mock hierarchy:

KBSecret::Generator::Abstract
  - KBSecret::Generator::Basic # the current `SecureRandom` implementation
  - KBSecret::Generator::Whatever

This would necessitate a configuration change, from:

:generators:
  :default:
    :format: hex
    :length: 16

to:

:generators:
  :default:
    :type: basic
    :options:
      :format: hex
      :length: 16

But this could be checked for, for backwards compatibility.

[woodruffw]

Another idea: move the generator code out of KBSecret, and allow commands like kbsecret new to call an external command that generates the password. This would allow users to leverage the existing password generator ecosystem (pwgen, diceware, etc) and would simplify the codebase.

Pros:

• Keeps codebase simple
• Leverages existing tools
• Conceptually simple and simpler to configure (no more kbsecret generator new?)

Cons:

• Relies on the presence of external utilities
• Might make hinting the generator more difficult (e.g., telling it to use at least one number, one symbol, etc.)