feature request: add base64 variable modifier

Hi,

It would be very convenient to have a base64 variable modifier. Main use-case is to base64-encode passwords when changing your hashing scheme.

With a base64 encoding modifier (e.g. %B) something like this would safely work: userdb { driver = prefetch default_fields = plain_pass=%Bw } without things breaking when there are %-characters in the password, and without having to send the plaintext password to the database, as current solutions propose, which is never a good idea.

alternative solution: adapt the generic hash-modifier to accept a none/plaintext hashing-scheme and use the base64 output functionality that is already available there.

Thanks! Alex