bash aliases and functions in the commands plugin

It would be nice if they were updated periodically with the
PeriodicRescanner thing, but I'm not sure how to do that when it works
through a TextSource.
commands: use interactive bash shell for load/use aliases and functions
utils: fix crasing glib.spawn_async when envp=None

glib.spawn_async expect sequuence of strings as envp parameter.
Using "" instead of None prevent crashing.