implement tab completion for ch-image modify

[lucaudill]

#1408 adds a new subcommand to ch-image to modify existing images in storage. Implementing tab completion for this feature is somewhat non-trivial, and since the feature itself already presents a significant change in the functionality of ch-image I'm deferring this until the PR is merged. The non-triviality comes from the fact that the feature adds additional context to the ch-image CLI that will need to be taken into account when suggesting completions. Namely, we will now need to identify image references when parsing CLI. Note that this is already something we do for ch-run completion.