Add support for --unit-graph unstable option

[morr0ne]

This pull request adds support for the unstable cargo feature --unit-graph.
Currently this is just a draft since a lot still needs to be added before it's ready to merge:

• Add missing field not documented on the official docs.
• Implement convenience methods like those found in Metadata.
• Allow building the command by mimicking the MetadataCommand api.
• Feature-gate this since it relays on an unstable cargo features.
• Maybe merge this with Split main code into modules #192 for better code organizations.