Crontab environment variables

I've submitted https://github.com/ansible/ansible-modules-core/pull/923 for more complex handling of crontab environment variables. I made it
a separate module from cron as I wasn't sure of the best way of
unifying the syntax. I wanted stuff like this:

   - cronvar: name="EMAILPROG" value="/usr/sbin/sendmail"
   - cronvar: name="EMAILCMD" insertafter="EMAILPROG" value="$EMAILPROG doug"

I'm not sure how to integrate this with the cron module in a way that
makes the syntax sensible without breaking changes to that module.

Doug