Add 'push' and 'pop' commands

Alas, they don't work as expected on OS X (what about Linux?)
I want them to change directory of the *host* process, now they
just print the directory to jump to.

Can still use it like this though:

  pushd `music-util push pitch`
  pushd `music-util push p`
  pushd `music-util push prel`
  pushd `music-util push art`
  etc.
v0.17
Comment in cabal file