Implements #46: Hooks to run container launch / shutdown tasks.

Launch arbitrary user-specified executables on the host system
when containers are launched or destroyed. Environment
variables are passed to executable. Arguments may be specified
in deimos.cfg. Please see example.cfg.
Code in Deimos should be 79 columns wide (so here `/usr/local/bin/container_registry` -> `container_registry` since `subprocess` uses `execvp` anyways).