Import/export support

There's some "universal" formats being discussed elsewhere,

https://github.com/Authenticator-Extension/Authenticator/issues/282

https://github.com/helloworld1/FreeOTPPlus/issues/30

https://github.com/beemdevelopment/Aegis/issues/138

I personally quite like one otpauth URI per line plaintext format, and I can try implementing support for it. JSON schema is bit "unknown" at this point however, so I am not excited about adding support for all off them.