A new filter to programatically edit the tail end of the commit log
messages.

* cc/interpret-trailers:
  Documentation: add documentation for 'git interpret-trailers'
  trailer: add tests for commands in config file
  trailer: execute command from 'trailer.<name>.command'
  trailer: add tests for "git interpret-trailers"
  trailer: add interpret-trailers command
  trailer: put all the processing together and print
  trailer: parse trailers from file or stdin
  trailer: process command line trailer arguments
  trailer: read and process config information
  trailer: process trailers from input message and arguments
  trailer: add data structures and basic functions