Update bash completion

- Refactor options completion to remove a little bit of code
  duplication.
- Add subcommand completion for the snapshot command. It is not perfect,
  but it's a start.
- Add some comments to functions here and there.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>