Add compatibility for totp codes with different intervals

[gergesh]

From my testing, everything seems alright but it could be that I haven't tested everything.
The downside is that the ~/.2fa file format has changed a bit, so you'll have to (very slightly) modify it to migrate to the new version.

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[gergesh]

Signed.

[googlebot]

CLAs look good, thanks!

[dmitshur]

This isn't gofmted.

[gergesh]

Done.

[xplshn]

@gergesh could you adapt to https://github.com/xplshn/2fa? I'm going to maintain my fork even if Russ doesn't merge its changes, and its going to be available via dbin too