Request for support for backup codes

[tedwu13]

Hey contributors,

Our company has been using this gem to handle two factor authentication. Is there going to be support for generating backup codes in cases when our users lose access to their phones? I've noticed there were some attempts to do that in the past (https://github.com/Houdini/two_factor_authentication/pull/92). Can we restart and continue the conversation on implementing backup codes? @Houdini @sbc100 @monfresh

Let me know how I can help! Thanks for the great work for this gem