Add a way to not append backport version number

In some cases, package dependencies might prevent installing versions
backported from the next distribution. In that case you can opt to not
change the version of backported package, but take care to replace it
with a correct official package in the future to avoid any problems with
dependencies.
Merge pull request #1 from drybjed/no-backport-version

Add a way to not append backport version number