Add support for issuer in google-authenticator otpauth:// URL.

As per issue #391, allow to specify a `&issuer=<issuer>` in `otpauth://` generated URL done by google-authenticator. By default it'll use the hostname. Setting it to an empty value like --issuer='' prevents &issuer= to be appended to the URL all together.

This is in line with the wiki page on https://github.com/google/google-authenticator/wiki/KeyUriFormat recommending to use both an issuer label prefix and an issuer parameter.