feature request: add base64 variable modifier

[alex]

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