exec with variables as arguments should still work

[nfischer]

Related to #23, CC @mikeerickson

The translator currently doesn't properly handle exec commands that expand variables. Something like:

branchName=master
git checkout $branchName
printf $PATH

should translate into:

var branchName = master
exec('git checkout ' + branchName);
exec('printf ' + env.PATH); // or exec('printf $PATH');