Allow mocking `time`

[colemickens]

When I looked, it didn't seem like it was possible to mock the various usages of time.Now making it hard to do nice testing around this library.

Thanks!

[kisom]

Thanks, @colemickens. I've got a PR open to support mocking the clock.

[colemickens]

Thank you, especially for the very fast turnaround. Cheers!

[colemickens]

Actually, I don't really understand how this is usable. It's not exported and there's no setter functionality. I think this should be reopened.

[kisom]

I misunderstood - you want to be able to test this from an external package. I've included support for that now in #17.

[dominikschulz]

Also this change seems to break TOTP. AFAIK the clock needs to be initialised.