Add cleanupCmds option to pkgs.myEnvFun

This option allows the user to specify commands to run when the shell
exits. This can be used to cleanup operations done in `extraCmds'.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Merge pull request #464 from the-kenny/my-env-exit-commands

Add cleanupCmds option to pkgs.myEnvFun. The downside is that /bin/sh remains in memory to be able to execute cleanup commands.