add a "step" argument to range()

This patch adds an optional "step" argument to the stdlib range()
function.  There is no change to the default behavior of the function;
however, passing a numeric "step" argument invokes the Ruby Range#step
method, e.g.

    range("0", "9", "2")

returns

    [0,2,4,6,8]

I've created ticket [13601](https://projects.puppetlabs.com/issues/13601) 
in Redmine to track this request.

-steve