bzr shell completion

[Martin Pool]

I've just merged in the small zsh shell completion function from Clint
Adams, which lets zsh complete bzr command names.  The shell-complete
command ought to also allow completers to show a list of valid options
for any command.

The change to define options by arguments should make it easy for
shell-complete to describe which options take arguments, and what
arguments are allowed.  We might need to add more specificity to some
option objects.

--
Martin