Add option for configuring the length of the passphrase

[Mathias Jean Johansen]

At the moment, passphrase is hardcoded to generate only six-word passphrases, but in some cases, you might want to generate a shorter or longer passphrase. It would be great if the application could handle this so we could configure the length of our passphrase this way:

% stack exec -- passphrase -n 10

Running the command above should generate a passphrase of 10 words rather than six. It needs to handle situations where the provided input is not a (positive) number and then fall back to six as its default value.