TOTP to_uri doesn't support issuer label prefix
*Created originally on Bitbucket by Anonymous*
From the [TOTP key URI spec](https://github.com/google/google-authenticator/wiki/Key-Uri-Format#label), it seems you can prefix the `label` field with the issuer *in addition* to providing the issuer parameter:
> We recommend using both an issuer label prefix and an issuer parameter, described below.
The current `to_uri` method does not format the `label` this way by default if `issuer` is set and also actively prevents you from including a `:` in the `label` field.
Happy to submit a PR adding this capability; not sure what the best API is. Additional flag to `to_uri` perhaps? Or always including it if `issuer` is set?
issue