New CLI command

Subcommands should always be preferred over adding more top-level commands.

Code flow for commands is generally:

CLI (command/) -> API Client (api/) -> HTTP API (command/agent) -> RPC (nomad/)

Code

Consider similar commands in Consul, Vault, and other tools. Is there prior art we should match? Arguments, flags, env vars, etc?
New file in command/ or in an existing file if a subcommand
Test new command in command/ package
Implement autocomplete
Implement -json (returns raw API response)
Implement -verbose (expands truncated UUIDs, adds other detail)
Update help text
Implement and test new HTTP endpoint in command/agent/<command>_endpoint.go
Register new URL paths in command/agent/http.go
Implement and test new RPC endpoint in nomad/<command>_endpoint.go
Implement and test new Client RPC endpoint in client/<command>_endpoint.go (For client endpoints like Filesystem only)
Implement and test new api/ package Request and Response structs
Implement and test new api/ package helper methods
Implement and test new nomad/structs/ package Request and Response structs