Allow TOTP generation in the future

For my use case (TOTP codes valid for 24 hours that I need to know the day before) I need to generate TOTP codes in the future, not only in the present, but [totp.gen()](https://github.com/guyht/notp/blob/master/index.js#L157) **explicitly** forbids this possibility.

Is there a reason for this limitation? Would it be possible to allow changing `_t` just like it is possible to change `time`?