Implement `-missing` flag for `dep status`

[darkowlzz]

dep status -missing should list the dependencies that are missing from the lock file but are used in the project.

Refer: command spec

Example code: #613

[adamo57]

is anyone looking into this? I would love to dive into it :)

[darkowlzz]

@adamo57 you're the first one. Go for it :)