Add issuer parsing and SteamGuard encoding

[tschwery]

These changes automatically parse the issuer if specified in the URL.

If the issuer is Steam, it uses the SteamGuard encoding that returns a code on 5 chars with a specific alphabet.

[qbit]

Is the SteamGuard encoding documented anywhere?

I am not particularly a fan of adding per issuer logic into the lib. Maybe an optional callback to the OTP function would be better suited to handle this sort of thing.

[tschwery]

No, it is not documented. It's more "examples floating around" than a proper documentation.
I can totally understand that you are not a fan of adding a specific, undocumented format to the library.

Yes, a callback could allow changing only a portion of the behavior (in this case generating the OTP string from the counter), I will look into this direction.

[kisom]

fwiw I don't really have a problem with this, but you could also take the same approach I did with Google TOTPs.

[qbit]

The google approach would also not change the function def. I’d be fine with that as well!