Create a syllable password generator

[robyoder]

We like Apple's syllable password generator, which produces 20-char passwords like wyvve4-jixzyH-rivkej. We should add a new recipe type that can produce passwords like that given a character length.

Right now, our word list generator only supports a word count, and since the words are different sizes, there is no guarantee about the actual length of the passphrase, but the entropy is guaranteed. With syllables that are all the same size (#17), we can be more creative in how we combine them (#18) and can generate a syllable password given any requested character length.

There are other things we may want to do for syllable passwords that we wouldn't want to do for word list passwords, like insert a random digit or symbol (#16), that would cause these two use cases to further diverge.