pluggable to execute outside command

[kibitan]

in order to allow users how to process the anonymization, introduce syntax, and call outside commands.

idea

users:
  email: `md5` # call shell command

it's passing value via stdin and take result from stdout

downside is performance regression and possible to failure

refs: @tonklon gave me the idea for this on 2014 Sep frankfurt.rb, thanks a lot!