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.
Nowhere else in Deimos do we use backslash line continuation and it would be nice to keep it that way. How about this?

```python
onl = self.onlaunch.override(onlaunch)
ond = self.ondestroy.override(ondestroy)
return onl, ond
```

Sorry to push this commit back on the queue. I really appreciate your patience with my comments and look forward to merging in your contribution.