Implement Check Command

[theory]

This command runs some sanity checks and outputs a report on what it found. Some of the checks I'm thinking it should have:

• Steps missing from the plan
• Dependency chain problems (cycles, missing dependencies?)
• Invalid plan file formatting
• Invalid config file formatting
• Missing step and tag deployments in the database
• Missing tags in the database
• Disagreements as to deployment order between the plan and what is deployed to the database

I'm sure we can think of more things as time goes on. Maybe not all will run by default, some turned on by --enable- options or -all. Or maybe we just always run everything.