Add a meta get command

[craigds]

Description

I added a meta command that lets you view the meta/ blobs in the repository.

In future we'll add write commands (I'm thinking (add, replace, delete) that will allow changes to the meta blobs. Some blobs are required to read the repo features, so those will be blacklisted from modification.

Usage info:

$ sno meta --help
Usage: sno meta [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  get  Prints the value of meta keys for the given dataset.

$ sno meta get --help
Usage: sno meta get [OPTIONS] DATASET [KEYS]...

  Prints the value of meta keys for the given dataset. If no keys are given,
  all available values are printed.

Options:
  -o, --output-format [text|json]
  --json-style [extracompact|compact|pretty]
                                  How to format the JSON output. Only used
                                  with -o json

  --include-readonly / --exclude-readonly
                                  Include readonly meta items
  --help                          Show this message and exit.

Output

Related links:

This works towards #102

Checklist:

• Have you reviewed your own change?
• Have you included test(s)?
• Have you updated the changelog?