add non-interactive --diff mode

Call `rpmconf -D [-a|-opackage]` to non-interactively dump diff
of config files. This is useful to audit changes to one or more
packages after upgrade, whether via the CLI or via
unattended automation. See README for example usage.
Merge pull request #9 from jumanjiman/diff_mode

add non-interactive --diff mode
add note how to build package