Allows length of OTP to be configured

Allows length of OTP to be configured.

I'm not sure if this fits the roadmap, but sometimes an application may need a longer (or shorter) one-time password.
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> Set config values, if desired, for maximum second factor attempts count, allowed time drift, and OTP length.

>  ```ruby
  config.max_login_attempts = 3
  config.allowed_otp_drift_seconds = 30
  config.otp_length = 6
  ```