Merge pull request #13 from vincentbernat/feature/more

Update completion of various commands
Merge pull request #11 from vincentbernat/fix/none

Don't complete image names with <none>:<none>
Merge pull request #10 from vincentbernat/fix/multiple-names

Handle container with multiple names.
Use _call_program everywhere instead of calling docker directly

Since on some OS, docker is now called docker.io, this makes easier for
the user to customize the completion.