0.4.4: add deferring path completion to the shell

This release adds ShellPathCompletion - a class with a similar signature to Completion that exits and triggers the calling shell's built-in path completion functionality when run.

The new functionality in this version depends on an update to shell hooks, so you'll need to re-initialise any hook after upgrading to this version. Existing hooks will continue to work correctly except with the deferred path completion.