title: Rails has_secure_password change BCrypt cost factor (aka work factor)
body: <p>When using has_secure password in rails, how can I change the default cost factor BCrypt uses when creating the password digests?</p>
  
  <p>I'd like to do this because the default cost factor used -- 10 -- is apparently a bit low (<a href="https://stackoverflow.com/questions/6832445/how-can-bcrypt-have-built-in-salts">this post</a> recommends setting it to 12 at least). </p>