pushd and popd no longer print output automatically.

This is a little controversial since the shell itself _does_ print by default. But in my opinion it's just noise and if I really want to print out the dir stack I can just call dirs() so I don't really see why pushd / popd do it too.

In the shell I can pipe it to /dev/null but here in shelljs I cannot without resorting to exec('pushd ' + dir + ' > /dev/null') or something similar. I will do that if you reject this pull request but it would be nice if it was just better by default :smile: 

Optionally, if you would prefer I could add a silent option to pushd / popd and only pass -q on to _dirs if silent is passed. e.g. `pushd('-q')` and `popd('-q')`